Passage One

Why is Thomas Jefferson mentioned in this passage a. As an example of interracial intimacy. b. Because he was a slave owner and was ply against interracial marriages. c. Because he was one of the early advocates of interracial marriages. d. Because he married a black woman.

如图所示，发光点S向平面镜射出一细激光束，反射后在一光屏上得一光点S′。平面镜开始时在与光屏平行a处，后绕激光束的入射点O以角速度ω逆时针旋转。SO=h，当转到30⁰的位置b时,光斑S′在光屏上移动的瞬时速度的大小为                      

ω

A．6ωha O 30⁰

B．8ωhb

C．4ωh

D．2ωh S光屏 S′