17. Given the equation about $x$, $3 x-4=a$ and $\frac{x+a}{3}=1$, if the root of the former is twice the root of the latter, then the value of the constant $a$ is ( ).
(A) 0.
(B) 1.
(C) 2.
(D) 4.
Answer: C.
Solution: Solving the equation $3 x-4=a$, we get
$$
x=\frac{a+4}{3} .
$$

Solving the equation $\frac{x+a}{3}=1$, we get
$$
x=3-a \text {. }
$$

According to the given condition, we have
$$
\frac{a+4}{3}=2(3-a),
$$

Solving this, we get
$$
a=2 \text {. }
$$

Therefore, the correct choice is C.