When using a general LP model for transportation problems, if there are $4$ ?sources and $3$ ?destinations, which of the following statements is true?

a$.$

There are typically $7$ ?decision variables and $7$ ?constraints.

b$.$

There are typically $4$ ?decision variables and $3$ ?constraints.

c$.$

There are typically $12$ ?decision variables and $12$ ?constraints.

d$.$

There are typically $12$ ?decision variables and $7$ ?constraints.