题干
Canada, _____ second to Russia in size, is _____ home of all kinds of animals and plants, making it famous for its natural scenery.
A：/; a
B：the, the
C：the, /
D：/ ; the
答案(点此获取答案解析）
D
所示是用以说明向心力和质量、半径之间关系的仪器，球p和q可以在光滑水平杆上无摩擦地滑动，两球之间用一条轻绳连接，mp＝2mq，当整个装置绕中心轴以角速度ω匀速旋转时，两球离转轴的距离保持不变，则此时( ) 
