开门过程中的障碍检测触发指定次数故障“ERROR”LED灯闪烁（）。
A.11
B.2
C.4
D.6
参考答案：D
She promised to ______ at the meeting but she didn’t ______. [ ]
A. turn up; keep her word
B. present; keep her promise
C. attend; turn up
D. appear; keep her words
下列统计函数中不能忽略空值(NULL)的是( )。
A．SUM
B．AVG
C．MAX
D．COUNT
