Answer:
The correct answer is A. energy was required to produce the concentration gradient.
Step-by-step explanation:
There are two types of transport through which a molecule moves in and out of the cell and these are active transport and passive transport. In passive transport, the molecule moves without the help of energy because they move down there concentration. So in passive transport, if concentration gradient changes then passive transport will take place.
In active transport, the molecule move against their concentration gradient with the help of energy or ATP. So in passive transport, the concentration of molecule can be high in the cell because its transport occurred against the gradient with the help of energy. So the correct answer is A.