Question

Let S be the set of all a ∈ R for which the angle betwen the vectors
u = a (logₑb) i − 6 j + 3 k and
v = (logₑb)i + 2j + 2a(logₑb)k , (b > 1) is acute. Then S is equal to ___

A. [infinity], - 4/3
B. ∅
C. (-4/3 , 0)
D. (12/7, [infinity] )