Everyone knows that _______ is dangerous to play with fire, but _______ is difficult is to prevent children from playing with fire.

A．it, it

B．what, what

C．it, what

D．what, it

挡土墙墙身砌筑应采用（），确保灰缝饱满；砌块应大面朝下，丁顺相间，互相咬接，上下错缝，不得有（）；墙身排水孔严禁倒坡。