Write the Function in the Form Y = F(u) and U

Multiple Choice

Write the function in the form y = f(u) and u = g(x) . Then find dy/dx as a function of x.
- $$y = \cos ^ { 4 } x$$

A) $y = \cos u ; u = x ^ { 4 } ; \frac { d y } { d x } = - 4 x ^ { 3 } \sin \left( x ^ { 4 } \right)$
B) $y = u ^ { 4 } ; u = \cos x ; \frac { d y } { d x } = - 4 \cos ^ { 3 } x \sin x$
C) $y = u ^ { 4 } ; u = \cos x ; \frac { d y } { d x } = 4 \cos ^ { 3 } x \sin x$
D) $y = \cos u ; u = x ^ { 4 } ; \frac { d y } { d x } = - \sin \left( x ^ { 4 } \right)$