A Sample Containing $4.0 \times 10 ^ { 18 }$ Atoms of a Radioactive Isotope Decays with a Half-Life Of Atoms

A sample containing $4.0 \times 10 ^ { 18 }$ atoms of a radioactive isotope decays with a half-life of $2.3$ years. How many undecayed atoms are left after $3.7$ years?

A) $1.7 \times 10 ^ { 18 }$
B) $1.3 \times 10 ^ { 18 }$
C) $2.5 \times 10 ^ { 18 }$
D) $1.1 \times 10 ^ { 18 }$