I have a lot of homework to do today, _____ I cannot go swimming with you.
A：because
B：so
C：then
D：but
—Dad, I'll be in the final singing competition tomorrow.
—Be careful not to get a sore throat and your voice.
宋朝海外贸易兴盛的主要原因是（ ）
根据“好奇”号火星车传同的图象推测火星可能存在过生命，主要依据是（ ）
