When using a general LP model for transportation problems if

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

There are typically 4 decision variables and 3 constraints.
There are typically 12 decision variables and 7 constraints.
There are typically 7 decision variables and 7 constraints.
There are typically 12 decision variables and 12 constraints.
   There are typically 12 decision variables and 3 constraints.

Solution

ANSWER

There are typically 12 decision variables and 7 constraints.

each source is a decision variable but each source have 3 destination so there are 4*3 = 12 decision variables

constraints

we have constraints of source ( 4) and constraints of destinations (3) in total are 7 constraints