Fill in the Missing Value $9.21 \mathrm {~cm} ^ { 2 } =$

Fill in the missing value.
- $9.21 \mathrm {~cm} ^ { 2 } =$ _________ $\mathrm { mm } ^ { 2 }$

A) 9210
B) $92.1$
C) 92,100
D) 921