Circular Functions - A question HELP!

[squance]

Hey. My sister needs help with a MC.

If sin a = -0.8 and cos a = -0.6, the value of cos((pi/2)-a) is :

A. pi/2 + 0.6
B. -0.6
C. -0.8
D. 0.6
E. 0.8

She thinks the answer is B but apparently the answer is C. Can anyone explain why?

[mark_alec]

cos(pi/2 - a) = sin(a)

If you draw a right angled triangle, you will see that one angle is a, the other must be (pi/2 - a), since the internal angles add up to pi.

[squance]

Thanks!! My sister gets it now