If A, B, C, and D, Are the Only Possible

If A, B, C, and D, are the only possible outcomes of an experiment, find the probability of D using the tablebelow. $\begin{array}{l|cccc}\text { Outcome } & \text { A } & \text { B } & \text { C } & \text { D } \\\hline \text { Probability } & 1 / 7 & 1 / 7 & 1 / 7\end{array}$

A) $4 / 7$
B) $1 / 7$
C) $1 / 4$
D) $3 / 7$