7.设函数 =arcsin (x)^2, 则 dy/dx= ()-|||-A. dfrac (2x)(sqrt {1+{x)^4}} B. dfrac (2x)(sqrt {1-{x)^4}}-|||-C. dfrac (1)(sqrt {1-{x)^4}} D. dfrac (1)(sqrt {1+{x)^4}}