Which of the following is not an identity?

a. cos^2 x csc x - csc x = -sin x

b. sin x(cot x + tan x) = sec x

c. cos^2 x - sin^2 x = 1- 2sin^2 x

d. csc^2 x + sec^2 x = 1

A ) cos² x · 1 / sin x - 1 / sin x = - sin x
cos² x - 1 / sin x = - sin x 
- sin² x / sin x = - sin x
- sin x = - sin x     ( correct )
B ) sin x ( cos x / sin x + sin x / cos x ) = 1 / cos x
sin x · ( cos² x + sin²x ) / sin x cos x = 1 / cos x
sin x · 1 / sin x cos x = 1 / cos x
1 / cos x = 1 / cos x  ( correct )
C ) cos² x - sin² x = 1 - 2 sin² x
1 - sin² x - sin² x = 1 - 2 sin² x
1 - 2 sin² x = 1 - 2 sin² x   ( correct )
D ) 1/sin²x + 1/ cos²x = 1
cos²x + sin² x / sin² x cos² x = 1
1 / cos² x sin² x = 1
cos²x sin² x ≠ 1
D ) is not an identity.