According to some experts, why do birth rates fall when a country’s economy’s improves?

One of the main reasons why the birthrates fall in the countries that have better economies is because the people focus much more on pursuing careers. With pursuing a career, the people tend to get married later, instead of in their 20's they get married in their 30's. Once they get married, because of their careers, they tend to postpone having a child, so by the time they do have a child, they are usually in their late 30's. This leaves very little time for having a second child as the women are fertile until their early to mid 40's, thus usually they only have one or two children, very rarely more then that.

the answer is b. 300t + 3/2xt

um is there supposed to be choices?