Answer: d)
Step-by-step explanation:
They have improved their economy by improving agriculture and because of it, they grew financially. By improved agriculture, we are considering a good quality of the crops which is rice (the most important) in Vietnam and it is growing around in the Red and Mekong river.
Other crops are corn, nuts, sweet potatoes and more. With the good quality of food, more people will want to buy them and that is where the economy rise starts.