4.(单选题,3.0分)-|||-曲面 z=f(x,y) 在点(x0,y0,z0)处的切平面法向量与Z轴夹角为锐角,则该法向量方向余弦中的cosY等于 () .-|||-A -1-|||-B 1-|||-C dfrac (1)(sqrt {{{f)_(x)}^2((x)_(0)(y)_(0))+({f)_(3)}^2((x)_(0),(y)_(0))+1}-|||-D dfrac (-1)(sqrt {{{f)_(x)}^2((x)_(0),(y)_(0))+({f)_(3)}^2((x)_(0),(y)_(0))+1}}