Normally when price per unit of a good falls, its

quantity demanded increases
quantity demanded decreases
quantity demanded remains constant
none of these happens

The correct answer is A.

The law of demand states that, all other things being equal, the quantity demanded of a good or service will decrease as the price of that good or service increases, and conversely, the quantity demanded will increase as the price decreases.

This is because, as the price of a good decreases, consumers have more purchasing power, and are therefore able to buy more of the good. Additionally, as the price of a good decreases, it becomes relatively cheaper compared to other goods, and consumers are therefore more likely to choose to buy that good instead of other goods.

There are a few exceptions to the law of demand. For example, if a good is a necessity, such as food or water, the quantity demanded may not decrease as the price increases. Additionally, if a good is a luxury, the quantity demanded may not increase as the price decreases.

However, the law of demand is generally a good predictor of how consumers will behave in response to changes in price.