What are three causes of the trans altantic slave trade

"A main cause of the trade was the colonies that European countries were starting to develop. In America, for instance, which was a colony of England, there was a demand for many labourers for the sugar, tobacco and cotton plantations. Paid labourers were too expensive, and the indigenous people had largely been wiped out by disease and conflict, so the colonisers turned to Africa to provide cheap labour in the form of slaves.  The first shipment of slaves from West Africa to the Americas, across the Atlantic Ocean, was in the early 1500s. European, Arab and African merchants were now selling humans as well as gold, ivory and spices."

answer: 25.8 years old or 25 years old

the answers are a b and d