How many ______ are there tomorrow?[ ]
A.flight
B.flights
C.fly
D.flies
B
两条对角线互相垂直的四边形是菱形。[ ]
揭穿或揭开煤层后,在石门附近()m范围内掘进煤巷时,必须加强支护。
A、15
B、20
C、30
D、50