Two Spaceships Are Traveling Past Earth at 90% of the Speed

Multiple Choice

Two spaceships are traveling past Earth at 90% of the speed of light in opposite directions (i.e.,they are approaching each other) .One turns on a searchlight,which is seen by scientists aboard the other.What speed do the scientists measure for this light (c = speed of light in a vacuum) ?

A) 1.9 c (equal to c + 0.9 c)
B) 1.8 c (equal to 2 × 0.9 c) .
C) c
D) 0.9 c