Solve the Problem $\mathrm { N }$ That Will Survive $x$ Hours Is Modeled by the Formula

Solve the problem.
-For a culture of 70,000 bacteria of a certain strain, the number of bacteria $\mathrm { N }$ that will survive $x$ hours is modeled by the formula $\mathrm { N } = 7000 \sqrt { 100 - \mathrm { x } }$ . After how many hours will 21,000 bacteria survive?

A) $91 \mathrm { hr }$
B) $97 \mathrm { hr }$
C) $79 \mathrm { hr }$
D) $9 \mathrm { hr }$