If the North Pole _____, the water would flow many cities.*meltedwould have meltedwould melthad melted

In this sentence, we need to choose the correct verb form to complete it meaningfully. The phrase "If the North Pole _____, the water would flow many cities" implies a conditional situation regarding the North Pole's status and its consequence regarding water levels.

1. Break Down the Problem
   The sentence is a conditional sentence in English, often structured with "if" clauses which can imply present, past, or hypothetical situations.

2. Relevant Concepts

   • "Melted" suggests a simple past action which may not conveys the intended conditional meaning.
   • "Would have melted" indicates a past event that did not occur, which does not fit the structure since it suggests the event has already happened.
   • "Had melted" is a past perfect form often used in third conditional sentences, but also suggests a completion prior to something else without linking adequately in this context.
   • "Would melt" indicates a future hypothetical outcome if the condition is fulfilled.

3. Analysis and Detail
   The correct verb form should reflect a hypothetical situation rather than a past completion. The sentence aims to communicate a future condition if the North Pole melts. Thus, the necessary phrase should enable the construction to imply that the melting would cause the water to flow to cities.

4. Verify and Summarize
   The most coherent completion is "If the North Pole melts, the water would flow into many cities," suggesting that the melting of the North Pole is a future possibility that would have a significant impact.

Final Answer

The correct choice is "melts." The complete sentence reads: "If the North Pole melts, the water would flow into many cities."