The principles of ATFM are that major measures are taken _______

The principles of ATFM are that major measures are taken _______ aided by _______measures; holding would rather be conducted _______ than _______.

A.n flight, preflight, in the air, on the ground

B.reflight, in flight, in the air, on the ground

C.in flight, preflight, on the ground, in the air

D.preflight, in flight, on the ground, in the air

正确答案是D