Use the formula for the cosine of the difference of two angles to find the exact value of the expression cos ( 3π /4 - π /6 )
Rewrite the expression using a sum or difference formula. Choose the correct answer below.

A. sin π /6 cos π /6 -sin 3π /4 cos 3π /4
B. sin 3π /4 cos π /6 +cos 3π /4 sin π /6
C. cos 3π /4 cos π /6 +sin 3π /4 sin π /6
D. cos 3π /4 cos π /6 -sin 3π /4 sin π /6