Which of the following sets of nuclei represents a triad of isotones?

• A
  $_{6}^{14}C$,$_{7}^{15}N$,$_{9}^{17}F$
• B
  $_{6}^{14}C$,$_{7}^{14}N$,$_{9}^{17}F$
• C
  $_{6}^{13}C$,$_{7}^{14}N$,$_{9}^{19}F$
• D
  $_{6}^{14}C$,$_{7}^{15}N$,$_{9}^{19}F$