The Lengths of Two Vectors a and B, and the Angle

Multiple Choice

The lengths of two vectors a and b, and the angle  between them, are given. Find the length of their cross product, $| \mathbf { a } \times \mathbf { b } |$ . $| \mathbf { a } | = 4 , \mathbf { b } \mid = 5 , \theta = 60 ^ { \circ }$

A) $20 \sqrt { 3 }$
B) $10 \sqrt { 2 }$
C) $10 \sqrt { 3 }$
D) $10$
E) none of these