A scientist has a sample of bacteria that initially contains 10 million microbes. They observe the sample and find that the number of bacterial microbes doubles every 20 minutes.

Which exponential function represents $M$, the total number of bacterial microbes in millions, as a function of $t$, the number of minutes the sample has been observed?
A. $M(t)=2 \bullet 10^{20 t}$
B. $M(t)=10 \bullet 2^{20 t}$
C. $M(t)=2 \cdot 10^{\frac{t}{20}}$
D. $M(t)=10 \cdot 2^{\frac{t}{20}}$