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 Then a
+b
r
r
is a unit vector if
(A)
θ =π4
(B)
θ =π3
(C)
θ =π2
(D)
π32
θ =
18 The value of 
ˆ
ˆ
ˆ
ˆ ˆ
ˆ
ˆ
ˆ
ˆ