Review the reading passage from Unit 7. Choose the best heading for paragraphs A-G from the list of headings (a-g).【请直接填写对应的小写字母】a.Blue holes teach us about the universeb.Some places yet to be mappedc.The risks of entering blue holesd.Underwater clues to our ancient paste.What and where blue holes aref.What blue holes are like insideg.Why people explore blue holes1. Paragraph A__________2. Paragraph B__________3. Paragraph C__________4. Paragraph D__________5. Paragraph E__________6. Paragraph F__________7. Paragraph G__________

A. to explain the importance of studying blue holes
B. to show why blue hole exploration is challenging
C. to describe the structure of a blue hole
D. to explain what blue holes are and how they are formed

A. to explain the importance of studying blue holes
B. to show why blue hole exploration is challenging
C. to describe the structure of a blue hole
D. to explain what blue holes are and how they are formed

A. to explain the importance of studying blue holes
B. to show why blue hole exploration is challenging
C. to describe the structure of a blue hole
D. to explain what blue holes are and how they are formed

A. to explain the importance of studying blue holes
B. to show why blue hole exploration is challenging
C. to describe the structure of a blue hole
D. to explain what blue holes are and how they are formed