Solve the Problem $S ( t )$ , in Millions of Dollars

Multiple Choice

Solve the problem.
-Yearly sales of an electronic device $S ( t )$ , in millions of dollars, $t$ years after 1997 can be estimated by $S ( t ) = 100 \cdot 6 ^ { t }$ . What is the doubling time for the yearly sales? Round your answer to the nearest tenth.

A) $0.7 \mathrm { yr }$
B) $0.4 \mathrm { yr }$
C) $3.0 \mathrm { yr }$
D) $0.8 \mathrm { yr }$