Read the passage about a climate agreement carefully and choose the preposition that correctly completes the phrase to bring the agreement into force ______ 2020, the first year of its proposed implementation.

Introduction / Context:
This question is another part of the same cloze passage on an international climate agreement. The sentence speaks about bringing the agreement into force in relation to the year 2020, which is described as the first year of its proposed implementation. The blank requires a preposition that accurately conveys the timing needed for the agreement to be effectively operational when that year begins. A correct understanding of how time related prepositions work in English is essential here.

Given Data / Assumptions:

• Sentence fragment: to bring the agreement into force ______ 2020, the first year of its proposed implementation.
• Options: on, before, at, previous.
• 2020 is treated as a point in time marking the first year of implementation.
• We want to express that the agreement should already be in force when 2020 starts.

Concept / Approach:
In English, we use before to indicate that something happens earlier than a specified time. If the agreement must be in force when 2020 begins, then the ratifications and formalities should be completed before 2020. The preposition on is typically used with specific days or dates, such as on Monday or on 1 January 2020. The preposition at is used for specific points like at noon or at midnight, not with a whole year. The word previous by itself is an adjective and cannot fill the blank after a preposition slot. Therefore, before is the preposition that correctly expresses the required timing.

Step-by-Step Solution:
Step 1: Identify the time reference. The year 2020 is given as the first year of implementation. Step 2: Understand the intention: the agreement should come into force earlier than this year so that implementation starts smoothly. Step 3: Evaluate on. It usually pairs with specific dates, not with an entire year alone in this sort of legal phrase. Step 4: Evaluate before. It naturally expresses earlier than 2020, which matches the requirement. Step 5: Evaluate at and previous, which do not form correct or complete expressions in this sentence.

Verification / Alternative check:
Read the sentence with each option. To bring the agreement into force on 2020 sounds incorrect because on is missing a specific day or date. To bring the agreement into force at 2020 is also incorrect as at rarely works with bare years. To bring the agreement into force previous 2020 is grammatically wrong since previous needs a preposition or a noun structure like previous to 2020. In contrast, to bring the agreement into force before 2020 is clear and standard, meaning earlier than the year 2020. This matches legal and journalistic usage relating to treaties and deadlines.

Why Other Options Are Wrong:

• on: Requires a specific day or date, for example on 1 January 2020, not just the year.
• at: Used for specific points in time like at midnight or at 5 p.m., not for an entire year in this context.
• previous: Functions as an adjective and cannot stand alone as a preposition here, so it fails grammatically.

Common Pitfalls:
Students sometimes choose on because they remember the pattern on 1 January 2020 and apply it incorrectly to a standalone year. Others may be tempted by at due to phrases like at the beginning of 2020, forgetting that the phrase here does not have an object after at. The best practice in cloze tests is to read the full phrase repeatedly and see which choice matches known standard expressions. In discussions of international agreements, the phrase before 2020 is often used to indicate a deadline, which aligns perfectly with this blank.

Final Answer:
The correct preposition is before, giving the phrase to bring the agreement into force before 2020, the first year of its proposed implementation.