$1 . a 、 b$ are skew lines, line $c$ forms an angle with $a$ equal to the angle it forms with $b$. Then the number of such lines $c$ is ( ).
(A)1
(B)2
(C)3
(D)infinite
1.(D).
Let the common perpendicular of $a$ and $b$ be $l$, then any line $c$ parallel to $l$ is perpendicular to both $a$ and $b$, hence there are infinitely many such lines $c$.