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答案：A
解析：
Choice A is correct. Recall that if θ is an angle measured counter-clockwise from the positive x-axis, the point, P, where the terminal ray of the angle intersects with the unit circle is P=(cosθ, sinθ).
To evaluate the sine and cosine, use a unit circle to see that the angle 7π/4 corresponds to the ordered pair (√2/2, -√2/2).
Since the y-coordinate of that ordered pair corresponds to the sine of that angle, then sin 7π/4 = -√2/2.
