The derivative of ( ddx( √1 - cosx21 - cosx2 ) ), ( x ∈ ( π2,π ) ) is equal to
a. ( 12√1 - cosx2 )
b. ( -12√1 - cosx2 )
c. ( 12(1 - cosx2)√1 - cosx2 )
d. ( -12(1 - cosx2)√1 - cosx2 )