—I haven't got a digital camera.

—I think you should buy _______. It's useful for you. [ ]

A. one

B. this

C. that

D. it

现代汽车都装备了专门的防盗系统，同时具备遥控、警报、防起动等功能。其中（）是最有效的防盗方式。

A.防起动系统

B.遥控

C.警报