碳三加氢反应器催化剂的活性可以用（）来表示。
A．反应器出口丙烯浓度
B．反应器消耗的氢气量
C．反应器进出口丙烯浓度差
D．氢炔比
参考答案：B
下列运算正确的是[ ]
A.a3×a=a4
B.（-a4）=a4
C.a2+a3=a5
D.（a2）3=a5
—What did your mother say on the phone?
—She asked ______.[ ]
A. why did I fail the exam
B. why I failed the exam
C. why did you fail the exam
D. why I fail the exam
