空气受热后体积变________，密度变_______，就会上升，周围的冷空气就过来补充形成风。
大；小
时钟9点30分时，分针和时针之间形成的角的大小等于（ ）
A．75°
B．90°
C．105°
D．120°
—Are Mr. and Mrs. Wang living alone in the house?
—Yes, altough they have three sons, ______ of them live with their parents. [ ]
A. neihter
B. both
C. all
D. none
