Solve the Problem $49 \mathrm {~cm} ^ { 2 }$ B) $21 \mathrm {~cm} ^ { 3 }$

Solve the problem.
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A) $49 \mathrm {~cm} ^ { 2 }$
B) $21 \mathrm {~cm} ^ { 3 }$
C) $343 \mathrm {~cm}$
D) $343 \mathrm {~cm} ^ { 3 }$