So if A, B, C and D are all positive numbers, then -ab -bc -cd is negative and ad is positive.

If they are all negative numbers, it is just the opposite of the case that -ab -bc -cd is negative and ad is positive.

If there are positive numbers and negative numbers, and negative numbers have odd numbers, then because each number is multiplied by the other two numbers, there must be negative numbers multiplied by negative numbers and negative numbers multiplied by positive numbers, and because there is a negative sign before three numbers, even if negative numbers are multiplied by positive numbers, there will still be positive numbers, so there must be a number that is positive.

Similarly, a negative number multiplied by a positive number may not have a negative sign to change the sign of the original product, so there must be a number that is negative.

If there are even numbers of negative numbers, there must be a negative product and a positive product. Because the appearance of the negative sign changes the original product, there must be a number that is negative and a number that is positive! ! !

I don't know if this answer is clear.