$[$Q$10\mathrm{.}9]$ Given a dictionary named "directory" and variables "last", "first", and "number", to what type category does the index expression, "last, first", in the following assignment statement belong?

directory$[$last$,$ first$]=$ number

Group of answer choices

b$)$ dictionary

a$)$ tuple

e$)$ None of the above

c$)$ index

d$)$ list