4.设 =(x)^3(y)^2+3, 则 dfrac ({partial )^2z}(partial ypartial x)= () .-|||-(A)6x^2y; (B)6xy^2; (C)6x^2y^2; (D) 6xy .-|||-5.设L为圆周 ^2+(y)^2=1, 则 int ((x)^2+(y)^2)dy= () .