Where can I find the video?

D is the correct answer because the author is comparing two groups that are not picked at random. We have one group which has no pets that are apparently happier than the other group with pets. The author concludes from this that if one wants to be as happy as possible then they should consider not getting a pet. But, what if the group that happens to have pets are just people who are sad in general? Maybe they all just went through a break up and then decided to get a pet to cope. Answer choice D points this out. What if people only get pets BECAUSE they are already sad and think that getting a pet will help them feel better. This would weaken the argument. So to answer your question, I think the answer choice would still be correct if it said "Some people who have pets feel happier because they have pets" because "some" can mean at least one or all.

"3 in a half minutes for the questions"

cries