Is - two billion dollars is a unit, one unit so it is singular and you use the singular verb
think - scientists is a plural, therefore, you must use the plural verb form of think
<u>Answer:</u>
You didn't include the answer choices, but a question with the same wording popped up on the quiz I just took and the answer for me was B : They suspect something strange may be going on.
I believe it would be " B "<span />
