When marginal is negative, it must be true that

The average is negative
The average is decreasing
The total is negative
The total is decreasing

The correct answer is: The total is decreasing.

Marginal is the additional amount of something that is gained or lost for each additional unit of something else. For example, if you are selling a product and your marginal profit is $10 per unit, then you are making an additional $10 profit for each additional unit that you sell.

Average is the total amount of something divided by the number of units. For example, if you sell 100 units of a product and your total profit is $1000, then your average profit is $10 per unit.

Total is the sum of all the amounts of something. For example, if you sell 100 units of a product and your total profit is $1000, then your total revenue is $1000.

When marginal is negative, it means that you are losing money for each additional unit that you sell. This can happen for a number of reasons, such as increasing costs or decreasing demand. When marginal is negative, it is likely that the average will also be negative, but this is not always the case. For example, if you have a large initial investment, your average profit may be positive even though your marginal profit is negative.

However, if marginal is negative for a long period of time, it is likely that the average will eventually become negative as well. This is because the losses from the additional units will eventually outweigh the profits from the earlier units.

When the total is decreasing, it means that you are losing money overall. This can happen for a number of reasons, such as increasing costs or decreasing demand. When the total is decreasing, it is likely that the average will also be decreasing, but this is not always the case. For example, if you have a large initial investment, your average profit may be positive even though your total profit is negative.

However, if the total is negative for a long period of time, it is likely that the average will eventually become negative as well. This is because the losses from the earlier units will eventually outweigh the profits from the later units.