A:to watch
B:to be watched
C:watching
D:being watched
____
求证:四边形ADCE是菱形.
如图,在三棱锥P-ABC中,∠APB=∠BPC=∠APC=90°,M在△ABC内,∠MPA=60°,∠MPB=45°,则∠MPC的度数为( )