A surveyor wishes to find the distance across a river while standing on a small island. If she measures distances of $a=30m$ to one shore, $c=60m$ to the opposite shore, and an angle of $B=100\degree$ between the two shores, find the distance between the two shores.

• A. ^$69.4m$
• B. $67.1m$
• C. $90.6m$
• D. $71.6m$