Question

1. which sentence has the correct subject - verb agreement?

either too much rain or too much snow makes the level of rivers rise.
either too much rain or too much snow make the level of rivers rise.

Explanation:

To determine correct subject - verb agreement with "either...or", the verb agrees with the nearest subject. Here, the nearest subject to the verb is "too much snow" (uncountable, singular), so the verb should be singular ("makes"). The first sentence uses "makes", while the second uses "make" (plural, incorrect).

Answer:

Either too much rain or too much snow makes the level of rivers rise.