从所给的四个选项中，选择最合适的一个填入问号处，使之呈现一定的规律性。（）

A．A

B．B

C．C

D．D

They _________ him unless he did as they asked. 　　[ ]

A. threatened killing 　　　

B. threatened to kill

C. were threatening to kill 　　

D. threated to kill