Question

the people who study rays think that rays
a help keep oceans clean.
b can make different sounds.
c are a danger to people.
d lived 150 million years ago.

Explanation:

Rays play a role in maintaining ocean - ecosystem balance, which includes helping keep oceans clean by controlling populations of their prey and participating in nutrient cycling. There is no evidence that rays can make different sounds as stated in option B. While some rays have venomous stingers, they are not generally a constant danger to people as option C suggests. And there is no basis for the claim that rays lived 150 million years ago as in option D.

Answer:

A. help keep oceans clean