The children all looked _____ at the broken model plane and felt quite _____.

A．sad, sad

B．sadly, sadly

C．sad, sadly

D．sadly, sad

大体积混凝土浇筑完毕后，后在( )内加以覆盖和浇水。

A．8 h

B．12 h

C．36 h

D．24 h