活化剂的作用是提高()的活性。
A.硫化剂;
B.促进剂;
C.补强剂;
D.防老剂。
参考答案:B
He didn't pass the exam _____ he had studied harder.[ ]
A. because
B. but
C. even if
D. if
流入节点的电流之和恒()于流出节点的电流之和。
A.大
B.等
C.小