16．已知四棱锥P−ABCDP-ABCDP−ABCD，底面ABCDABCDABCD是菱形，∠BAD=60∘\angle BAD=60\circ∠BAD=60∘，PD⊥PD\botPD⊥底面ABCDABCDABCD，且PD=CD=2PD=CD=2PD=CD=2，点MMM，NNN分别是棱ABABAB和PCPCPC的中点．（Ⅰ）求证：MN//MN//MN//平面PADPADPAD；（Ⅱ）求三棱锥D−PBND-PBND−PBN的体积．
