1.You'd better _____because you'er gaining weight

1.You'd better _____because you'er gaining weight
A.on diet B.be in a diet C.on a diet D.go on a diet
2.—I usually take a walk at night.
—_____do that again
A.Not better B.Better not C.Not better to D.Better not to
3.I'm _____weight.The doctor has warned me to keep off sugar.
A.taking up B.putting on C.putting out D.taking off
4.We all write_____,even when there is not much to say.
A.now and then B.by and byC.step by step D.more or less
5.He strongly advised us_____here a bit longer,but I advised_____.
A.to stay;to leaveB.staying;leavingC.to stay;leavingD.staying;to leave
6.Would you slow down a bit,please?I can't_____you.
A.keep up with B.put up with C.make up to D.hold on to
麻烦大家给我详细的解题语法,写出选择答案的原因.