In this English grammar question, choose the correct verb form to complete the sentence "The higher you climb, the more difficult it _____ to breathe."

Introduction / Context:
This question evaluates your understanding of verb tense and the use of present simple in general truths. The sentence "The higher you climb, the more difficult it _____ to breathe" describes a general fact about what happens as altitude increases. In such cases, English normally uses the simple present tense. Your task is to choose the verb form that correctly completes this comparative structure and expresses a general truth clearly.

Given Data / Assumptions:

• The sentence is "The higher you climb, the more difficult it _____ to breathe."
• The options are became, becomes, has become, and is becoming.
• The statement describes a general rule about height and breathing difficulty, not a single past event.
• The structure "The higher you..., the more..." typically pairs with simple present verbs.

Concept / Approach:
In English, when we describe facts that are generally true at all times, such as scientific facts or universal truths, we use the simple present tense. The comparative correlative structure "The higher you climb, the more difficult it becomes" is a standard pattern. Using past tense ("became") or present perfect ("has become") would wrongly suggest a specific time period. The present continuous ("is becoming") would imply a temporary, ongoing change rather than a general rule.

Step-by-Step Solution:
Step 1: Recognize that the sentence describes a scientific or physical fact about climbing higher and breathing. Step 2: Recall that universal truths and general facts are usually expressed in the simple present tense. Step 3: Test "became": "The higher you climb, the more difficult it became to breathe." This sounds like a narrative about a past event, not a general rule. Step 4: Test "becomes": "The higher you climb, the more difficult it becomes to breathe." This correctly expresses a general truth in present simple. Step 5: Test "has become": "The higher you climb, the more difficult it has become to breathe." This incorrectly suggests that some recent change has occurred, which is not intended. Step 6: Test "is becoming": "The higher you climb, the more difficult it is becoming to breathe." This continuous form does not fit a timeless law and sounds awkward. Step 7: Conclude that "becomes" is the most appropriate verb form.

Verification / Alternative check:
You can confirm the pattern by looking at standard examples of similar comparative structures: "The harder you work, the luckier you get" or "The more you read, the more you learn." In each case, the verbs "get" and "learn" are in the simple present tense. Replacing them with past or continuous forms would spoil the general, all-time truth. Similarly, "The higher you climb, the more difficult it becomes to breathe" follows the same pattern and sounds natural to native speakers.

Why Other Options Are Wrong:
"became" changes the sentence into a description of a completed past event, which conflicts with the general truth being expressed. "has become" suggests a recent change or development, as if breathing difficulty has only now started to increase, which is not the intended meaning. "is becoming" focuses on a process happening currently and temporarily, rather than on a timeless rule.

Common Pitfalls:
A common mistake is to overuse continuous or perfect tenses because they sound more advanced. However, in many exam questions, the simple present is the most accurate and elegant choice, especially for scientific facts and habitual actions. Another pitfall is to ignore sentence structures like "The more..., the more..." which almost always take simple present verbs. If you remember this structure, you can quickly eliminate the incorrect options and confidently choose the right one.

Final Answer:
The correct verb form is becomes, so the complete sentence is "The higher you climb, the more difficult it becomes to breathe."