Choose the correct answer in Exercises 16 to 19 16 If θ is the angle between two vectors 
a and 
rb
r
, then 
0
a b
⋅
rr≥
 only when
(A) 0
< θ <π2
(B) 0
π2
≤ θ ≤
(C) 0 < θ < π
(D) 0 ≤ θ ≤ π
17 Let   and 
a
rb
r
 be two unit vectors and θ is the angle between them