____ students have graduated from this school in the last twenty years.

A．Tens of thousands of

B．Tens upon thousands of

C．Tens in thousands

D．Ten thousands of

在四冲程柴油机中，发生缸内废气倒灌进气管的原因是（）。

A．进气阀提前开启角太小

B．进气阀提前开启角太大

C．排气阀提前开启角太小

D．排气阀提前开启角太大