题干
The yellow coat is as as the purple one.
A：cheap
B：heavier
C：cheaper
答案(点此获取答案解析）
A
The yellow coat is as as the purple one.
A：cheap
B：heavier
C：cheaper
，则在此过程中发出的光子总数是：
，
，已知
∈B,且B中含有3个元素，则集合B有( )