Solve the Problem $B ( x )$ Of a Dollar $x$ Years from Now Will Decrease According to the Formula

Solve the problem.
-An economist predicts that the buying power $B ( x )$ of a dollar $x$ years from now will decrease according to the formula $\mathrm { B } ( \mathrm { x } ) = 0.78 ^ { \mathrm { x } }$ . How much will today's dollar be worth in 4 years? Round the answer to the nearest cent.

A) $2.95
B) $3.12
C) $0.37
D) $0.91