The correct answer is the first option.
The sentence that presents the correct subject-verb agreement is the first one since in it we are talking about "heavy rains" OR "fast-melting snow". In other words, the subject is coordinated by the conjunction "OR" which makes the subject singular. The verb "causes" is conjugated in the third person singular.
The intrinsic eye muscles, or the <span>sphincter pupillae.</span>
5--all of the other sentences are details relating to this main point.
