情景匹配
⑴How many seasons are there in a year?____ A. Four
⑵What about some milk?____ B. She's in the kitchen.
⑶What would you like?____ C. They're on the sofa.
⑷Where are my warm clothes?____ D. Sounds good.
⑸I can't find Mum.____ E. I'd like a hamburger.
a1、a2
、a3…组成的新数列
a1+a4， a2+a5， a3+a6…是（ ）