You can go by these shorter examples. * Who is he? => Who do you think he is? / Do you know who he is? * Who are you? => Who do you think you are? (It's a common phrase as well as the title of a TV series) (The main question is expressed by "do you think" and the remainder is just filling in the detail of the question, so it doesn't have to be in question-like order anymore) Likewise, * What is the most serious pollution? => What does Jack think the most serious pollution is?

Hello. Great question! Here's my response: This sounds better: "B. What does Jack think the most serious pollution is?" Here're more explanations/examples. If you use "do", then either word order is fine: What DO + 3RD PERSON (plural) + think IS + _____? Example: What do they think is the problem? What DO + 3RD PERSON (plural) + think + _____ + IS? Example: What do they think the problem is? If you use "does", then this order sounds a lot better: What DOES + 3RD PERSON (singular) + think + _____ + IS? Example: What does Jack think the most serious pollution is? Example: What does mom think the answer is? Hope I helped a bit. Let me know if you still have questions. :)

the answer is B

I guess the first sentence leaved out "which". Isn't it???

i think the first one is right. The second one sounds weird