A:short
B:long
(Ⅰ)求证:平面AEC⊥平面PBC;
(Ⅱ)求直线AE与平面PAC所成角的大小.
—How about _______ in the river with us?
—Sorry, I can't. My parents often tell me _______ that.
大气中90%的氧气是下列哪类植物提供的( )