A．氧气和臭氧
B．乙酸和醋酸
C．正丁烷和异丁烷
D．甲烷和丙烷
答案：D
看图，判断下列句子正误，正确的在括号里打“√”，错误的打“×”。
( ) 1. The sun is shining in Picture 2.
( ) 2. There is a rainbow in Picture 1.
( ) 3. It's rainy in Picture 1.
( ) 4. The squirrel is drawing in Picture 2.
( ) 5. There are some clouds in Picture 1.
期前收缩后出现的代偿性间歇,是由于窦房结少发放了一次节律性冲动
