10.[半题题-|||-球面 ^2+(y)^2+(z)^2=1 与球面 ^2+((y-1))^2+((z-1))^2=1 所围立体的公共部分在xoy面-|||-上的投影是 ()-|||-(A) ^2+2((y-dfrac {1)(2))}^2=dfrac (1)(2) (B) ^2+2((y-dfrac {1)(2))}^2leqslant dfrac (1)(2) .-|||-(C) ))^2=dfrac (1)(2) z=0 .