Triangle ABC was transformed using the rule (x, y) → (–y, x). The vertices of the triangles are shown.

A (–1, 1)
B (1, 1)
C (1, 4) A' (–1, –1)
B' (–1, 1)
C' (–4, 1)
Which best describes the transformation?

The transformation was a 90° rotation about the origin.
The transformation was a 180° rotation about the origin.
The transformation was a 270° rotation about the origin.
The transformation was a 360° rotation about the origin.