液化石油气的主要成分是（）。
A.丙烷、戊烷；
B.丙烷、丙烯、丁烷、丁烯；
C.丁烯、戊烷；
D.丙烯、戊烷
参考答案：B
In our class, when the bell rang and the teacher closed his book, it was a ______ for everyone to stand up.
A．signal
B．chance
C．mark
D．measure
成烃坳陷
