Solve Express the Result in Scientific Notation $6.43 \times 10 ^ { - 7 }$

Solve. Express the result in scientific notation. Round to the nearest hundredth, if necessary.
-If the density of a substance is $6.43 \times 10 ^ { - 7 }$ tons per cubic foot, what would be the mass of $6.13 \times 10 ^ { - 7 }$ cubic feet of this substance. (Use the formula D $\left. D = \frac { M } { V } . \right)$

A) $3.94 \times 10 ^ { - 14 } \text { tons }$
B) $3.94 \times 10 ^ { - 12 } \text { tons }$
C) $3.94 \times 10 ^ { - 13 } \text { tons }$
D) $0.39 \times 10 ^ { - 13 } \text { tons }$