腐蚀介质在疲劳断裂过程中的主要作用是（）。
A.形成疲劳源
B.扩展裂纹
C.导致脆断
D.导致塑性变形
参考答案：A
― Have you finished your composition already, Jack?
― Yes, I _____ it within half an hour. [ ]
A. have finished
B. finish
C. finished
D. had finished
樟脑、冰片、薄荷脑等受力易变形的药物宜用（）.
A、干法粉碎
B、加液研磨法粉碎
C、水飞法粉碎
D、以上均不可
