1, D. You can substitute it and find that only option D meets the requirements.

2.b continuity means that f(x) has a definition in this domain, so continuity must have a definition and necessary conditions.

3, a When x tends to 0, sinx = X. The formula becomes ax? /x？ +b/x？ If b is not 0, then the second term is infinite, and the limit will not be equal to 2, then b=0, b/x? =0。 The formula becomes ax? /x？ =2，a=2 .