It is essential that your application form( ) back before the

It is essential that your application form( ) back before the deadline.
A、must be sent
B、would be sent
C、be sent
D、will be sent
【正确答案】：C
【题目解析】：It be adj. that 虚拟语气，从句要求用should+v.或者v.，因此答案为C。