桌子上有若干个大小相同的玻璃杯，里面水的平均高度是5厘米．后来又拿来一个同样的玻璃杯，里面水的高度是8.2厘米，这时这些玻璃杯里水的平均高度是5.4厘米．问原来桌子上有几个玻璃杯？

A pumproom is suspected of accumulating gases after a ventilation machinery breakdown. Where should the combustible gas indicator case be placed when testing the pumproom atmosphere for combustible gases?（）.

A.In the lower level of the pumproom

B.In the middle level of the pumproom

C.In the upper level of the pumproom

D.On the deck outside the pumproom