If `f_r(x),g_r(x),h_r(x),r=1,2,3` are differentiable function and `y=|(f_1(x), g_1(x), h_1(x)), (f_2(x), g_2(x), h_2(x)),(f_3(x), g_3(x), h_3(x))| then dy/dx= |(f\'_1(x), g\'_1(x), h\'_1(x)), (f_2(x), g_2(x), h_2(x)),(f_3(x), g_3(x), h_3(x))|+ |(f_1(x), g_1(x), h_1(x)), (f\'_2(x), g\'_2(x), h\'_2(x)),(f_3(x), g_3(x), h_3(x))|+|(f_1(x), g_1(x), h_1(x)), (f_2(x), g_2(x), h_2(x)),(f\'_3(x), g\'_3(x), h\'_3(x))|` On the basis of above information, answer the following question: Let `f(x)=|(x^4, cosx, sinx),(24, 0, 1),(a, a^2, a^3)|`, where a is a constant Then at `x= pi/2, d^4/dx^4{f(x)}` is (A) 0 (B) a (C) `a+a^3` (D) `a+a^4`