To show who is talking. Who specifically.
Answer:
A. A poll of one hundred students found that after participating in the food drive, seventy-three participants found the experience so rewarding that they sought out new volunteer opportunities.
Explanation:
EXPLAINS, IS
The tour guide EXPLAINS that either candles or a lantern IS used to see into the tomb.
Tour guide is singular. Therefore, verb used must be in singular form.
Lantern is singular and it is the nearest subject of the verb. Thus, the verb must also be in singular form.