$2.(40$ marks$)$ Consider these three relations and then write SQL statements to answer the questions that follow.

TRAVEL$\_$AGENT $($TA$\_$no$,$ name, age, salary$)$

CUSTOMER $($C$\_$ID$,$ name, departure$\_$city, destination, journey$\_$class$)$

TRANSACTION $($number$,$ C$\_$ID$,$ TA$\_$no$,$ date, amount$\_$paid$)$

a$.$ Compute the number of different customers who have a transaction