$A$ and $B$ are two sets having 3 elements in common. If $n(\mathrm{~A})=5, n(\mathrm{~B})=4$, then what is $n(\mathrm{~A} \times \mathrm{B})$ equal to?

• A

  0
• B

  9
• C

  15
• Correct answer
  D

  20

Hence, the correct answer is option: 'D'.