Write the Word or Phrase That Best Completes Each Statement $\mathrm { pH }$

Write the word or phrase that best completes each statement or answers the question.
-The hydrogen potential, $\mathrm { pH }$ , of a substance is defined by $\mathrm { pH } = - \log \left[ \mathrm { H } ^ { + } \right]$ , where $\left[ \mathrm { H } ^ { + } \right]$ is measured in moles per liter. Find the pH of a sample of lake water whose $\left[ \mathrm { H } ^ { + } \right]$ is $3.05 \times 10 ^ { - 9 }$ moles per liter. (Round to the nearest tenth.)

A) $6.4$
B) $10.1$
C) $8.5$
D) $7.3$