单项选择题：Smell the flowers before you go to sleep,and you may j

Smell the flowers before you go to sleep,and you may just -----sweet dreams.

A.keep up with B.put up with C.end up with D.catch up with

Sometimes our opinions differ ------ what we choos to observe and how we deal with what we've observed.

A.which B.since C.because D.because of

You ------ buy a gift,but you can if you want to.

A.must B.mustn't C.have to D.don't have to