Children aren't allowed _______ in the street.
A:playing football
B:playing the football
C:to play football
D:to play the football
在△ABC中,∠A、∠B、∠C的对边分别是a、b、c,且∠C=90°,c=10cm,a:b=3:4,求a、b的长.
青霉素是下列哪种真菌产生的( )