Write the Word or Phrase That Best Completes Each Statement $3.4 \times 10 ^ { 9 } \mathrm { yr }$

Write the word or phrase that best completes each statement or answers the question.
-The half-life of an element is $3.4 \times 10 ^ { 9 } \mathrm { yr }$ . How long does it take a sample of the element to decay to $\frac { 2 } { 5 }$ of its original mass? Use $A = A _ { 0 } ( 0.5 ) ( t / T )$ where $A _ { 0 }$ is the initial amount, $T$ is the half-life, and $t$ is the time. (Express results in scientific notation, rounded to the nearest hundredth.)

A) $2.16 \times 10 ^ { 9 } \mathrm { yr }$
B) $7.59 \times 10 ^ { 8 } \mathrm { yr }$
C) $4.49 \times 10 ^ { 9 } \mathrm { yr }$
D) $2.58 \times 10 ^ { 9 } \mathrm { yr }$