BCA makes more sense. The first mention of space mission (we don't know which one) - A space mission. The second mention of space mission (we now know which one) - THE space mission. I really don't think this is a good, reliable resource though, Molly.

17 is correct. c-b-a

Đúng rồi đấy bạn

BCA. is correct. for the reason Claire says. I was excited to join A space mission (any space mission). Then I prepared for many moths (we have to read into the text, that they he or she. worked hard for many months) to Join THE space mission (we have to read into the text they joined A space mission). They then refer to it THE space mission. The whole test is not very well laid or presented. There are thousands of test like these in China and Asia that are not really helpful.