Countries A and B shares their borders and have had a preferential trade agreement that eliminated customs, tariffs, and other barriers between the two countries. Countries A and B also came to an agreement recently to set up a common tariffs for goods/services imported to them. In addition, they decided to lift any regulation on investment and employment for each other's citizens. With this new set of agreements, what kind of transition have Countries A and B just made?

a. From free trade agreement to customs union

b. From free trade agreement to common market

c. From customs union to common market

d. From common market to economic union

please explain