某有机物的结构简式为,它可以发生的反应类型有(a)取代 (b)加成 (c)消去 (d)酯化 (e)水解 (f)中和 (g)加聚
A:(a)(c)(d)(f)
B:(b)(e)(f)(g)
C:(a)(b)(c)(d)(g)
D:除(e)(g)外
Of the three foreigners, one is from London, ____ are from the USA.
如图,在三棱锥P﹣ABC中,∠PAC=∠BAC=90°,PA=PB,点D,F分别为BC,AB的中点.
(1)求证:直线DF∥平面PAC;
(2)求证:PF⊥AD.