题干
I was listening to the music while _________ my homework yesterday evening.
A：to do
B：doing
C：did
D：is doing
答案(点此获取答案解析）
B
射线；铯137是铯133的同位素，半衰期约为30年，发生衰变期时会辐射
射线。下列说法正确的是
