-- Tu Youyou received the Nobel Prize for her ________ of anemisinin(青蒿素).
-- And she was the first Chinese who won the prize in science.
A:design
B:thought
C:invention
D:discovery
(Ⅰ)求证:PC⊥BC.
(Ⅱ)求二面角M﹣AC﹣B的大小.
如图所示的自动电路中,当开关S断开时,工作电路的情况是( )