推翻沙皇专制制度在俄国统治的事件是( )
①青藏高原____ A.地势平坦,一望无际
②内蒙古高原____ B.雪山连绵,冰川广布
③黄土高原____ C.地面崎岖,峰岭众多
④云贵高原____ D.千沟万壑,支离破碎
How many ______ are there under the table ?