烃类热裂解中二次反应有哪些？
参考答案：
二次反应主要有：烯烃的裂解；烯烃的聚合、环化和缩合；烯烃的加氢和脱氢；烃分解生碳。
At the beginning of class, the noise of desks __________ could be heard outside the classroom.
A．opened and closed
B．to be opened and closed
C．being opened and closed
D．to open and close
舌红绛而光者，属（）
A.阴虚
B.气虚
C.血虚
D.气阴两虚
E.水涸火炎
