1. If $a, b, c$ are distinct real numbers, and the coordinates of three points are $A(a+b, c), B(b+c, a), C(c+$ $a, b)$, then the positional relationship of these three points is ( ).
(A) on the same straight line
(B) form a right triangle
(C) form an obtuse triangle
(D) form an equilateral triangle