2: Both numerator and denominator are multiplied by 5. Obviously, the answer is 5.

3. from the basic limit formula sin x/x= 1, let x=3x, and the answer is C.

4. obviously, x=2 is undefined at 0, so when x is not equal to 0, f(x)=x-2, so it is obviously a decodable discontinuous B.

Simply put, it's all obvious.