He is __________ ping-pong.[ ]
A. play
B. playing
C. plays
答案:B
悬索桥刚架式桥塔通常采用( ),
A.T形截面 B.工字形截面 C.箱形截面 D.圆形截面
蓄电池测量电压时,必须在揭开电池盖()以后进行。
A.5MIN
B.10MIN
C.15MIN