1. If $a, b, c$ are three arbitrary integers, then $\frac{a+b}{2}, \frac{b+c}{2}, \frac{c+a}{2}$ are ( ).
(A) not integers
(B) at least one integer
(C) all integers
(D) at least two integers
-.1.C.
Notice that among three arbitrary integers $a, b, c$, at least two of them have the same parity. Without loss of generality, let these two numbers be $a, b$, then $\frac{a+b}{2}$ is an integer.