12. Starting from the number $a$, perform the following four operations:

Add 2, Subtract 2, Multiply by 2, Divide by 2, each operation only once, in any continuous order. For example, you can first multiply by 2, then subtract 2, etc. Then, the largest number that can be obtained at the end is ( ).
(A) $a$
(B) $a+1$
(C) $a+2$
(D) $a+4$
(E) It depends on whether $a$ is odd or even
12. (C) You cannot perform addition and subtraction consecutively, nor can you perform multiplication and division consecutively, otherwise the answer will only be a mess. To make the expression reach its optimal value, addition should be arranged after division and before multiplication. Therefore, the best expression is $\left(\frac{a-2}{2}+2\right) 2$ or $\left(\frac{a}{2}+2\right) 2$ $\cdots 2$.