Tom is always late for school. He ________ gets up early.
A:never
B:often
C:usually
D:sometimes
读中国主要山脉分布示意图,完成下列要求。
-- The Yangtze River.
质量为m的木块被水平力F紧压在倾角为θ=60˚的固定木板上静止,如图所示。则木板对木块的作用力大小为( )