If l1, m1, n1 and l2,m2,n2 are the direction cosines of two vectors and theta is the angle between them, then the value of costheta is
a.l1 l2 + m1 m2 + n1 n2
b.l1 m1 + m1 n1 + n1 l1
c.l2 m2 + m2 n2 + n2 l2
d.m1 l2 + l2 m2 + n1 m2​