--What do you think of it?

--I think it is a thing of importance that it ____ done soon.

A．is

B．is to be

C．be

D．to be

地坪层的垫层是地坪层的承重层，它必须有足够的强度和刚度，以承受面层的荷载并将其均匀地传给垫层下面的土层。