A young couple has three children. The probability that all three children are boys, if it is known that atleast one of the children is a boy is

• A

  _{$\frac{1}{4}$}

• B

  _{$\frac{1}{7}$}

• C

  _{$\frac{1}{8}$}

• D

  _{$\frac{1}{20}$}