Evaluate or Simplify the Expression Without Using a Calculator $\mathrm { pH }$

Multiple Choice

Evaluate or simplify the expression without using a calculator.
-The $\mathrm { pH }$ of a solution ranges from 0 to 14 . An acid has a pH less than 7 . Pure water is neutral and has a pH of 7. The $\mathrm { pH }$ of a solution is given by $\mathrm { pH } = - \log x$ where $x$ represents the concentration of the hydrogen ions in the solution in moles per liter. Find the $\mathrm { pH }$ if the hydrogen ion concentration is $6.4 \times 10 ^ { - 3 }$ .

A) $2.19$
B) $3.81$
C) $3.19$
D) $2.81$