有三个共点力，F1＝2N，F2＝4N，F3＝5N，它们的合力F的大小不可能是[ ]
A．0N
B．5N
C．7N
D．12N
答案：D
---Do you go to swim every day?
---No, but I ________.
A．used to do
B．used
C．used to do it so
D．used to
烧结加生石灰是为了消化后发挥粘结作用，促进颗粒成球。
