On the occasion of Diwali each student of a class sends greeting cards to others. If there are 20 students in a class, then how many greeting cards were exchanged?

Question

$\begin{array}{1 1} ^{20}C_2 \\ 2.^{20}C_2 \\ ^{20}P_2 \\2.^{20}P_2 \end{array}$