In this sentence improvement question, you must choose the option that best replaces the bracketed word ago in the sentence It has been a long (ago) I last saw you so that the sentence becomes grammatically correct and natural. Select the most appropriate alternative.

Introduction / Context:
This question focuses on improving a sentence by replacing an incorrect word with a more suitable one. The original sentence is It has been a long ago I last saw you, where the use of ago is wrong in this position. In natural English, we say It has been a long time since I last saw you to express that a considerable period has passed. Your task is to select the option that best fits into the structure after a long so that the sentence can be corrected easily.

Given Data / Assumptions:

• The sentence given is It has been a long (ago) I last saw you.
• The options to replace ago are time, since, before, and no improvement.
• The intended meaning is to say that a long period has passed since the last meeting.
• The structure It has been a long requires a specific noun to complete it.
• The exam expects you to identify the best improvement even if minor additional adjustments are implied.

Concept / Approach:
In English, ago is used with a simple past clause and a time expression, such as two years ago, and not with has been in present perfect structures. The correct natural pattern is It has been a long time since I last saw you. The key missing noun is time, which fits after a long and allows the rest of the sentence to be completed with since I last saw you. Among the given options, only time properly fills the blank, and the student can mentally supply since to complete the familiar pattern. The other options do not fit grammatically after a long in this structure.

Step-by-Step Solution:
Step 1: Notice that It has been a long ago I last saw you is incorrect because ago should not be used after has been. Ago normally follows a time phrase with simple past verbs. Step 2: Consider option time. Replacing ago with time gives It has been a long time I last saw you. Although we would usually say It has been a long time since I last saw you, time is the correct noun needed after a long. Step 3: Consider option since. It has been a long since I last saw you is ungrammatical because since cannot directly follow a long without a noun. Step 4: Consider option before. It has been a long before I last saw you does not make sense in standard English usage. Step 5: Consider option no improvement. This is clearly incorrect because the combination has been and ago in this slot is wrong. Step 6: Conclude that time is the best replacement for ago, allowing the well known pattern It has been a long time since I last saw you to be formed.

Verification / Alternative check:
Compare the corrected structure with standard English expressions. Native speakers regularly say It has been a long time since we met or It has been a long time since I visited that place. They do not say It has been a long ago we met. Only time fits naturally after a long in this kind of present perfect sentence. This confirms that time is the correct choice, even though the full ideal sentence also needs the word since later on.

Why Other Options Are Wrong:
since cannot directly complete the phrase a long since in this structure; it must follow a noun or time expression, as in a long time since.
before is used to refer to earlier points in time, but a long before I last saw you does not form a standard expression in this context.
no improvement is wrong because the given sentence is clearly ungrammatical with ago after has been.

Common Pitfalls:
Learners sometimes focus only on the general sense of past time and are tempted to retain words like ago or before, which are associated with earlier events, without checking the exact grammatical pattern. Another common mistake is overlooking the need for a noun like time after modifiers such as a long in present perfect constructions. Remembering standard phrases, such as It has been a long time since, will help in quickly identifying the correct improvement in similar questions.

Final Answer:
The most suitable replacement for ago is time, so that the sentence can be correctly interpreted as It has been a long time since I last saw you.