Each negative factor changes the sign of the multiplication. This means that an even number of negative factors gives a positive result, and an odd number of negative factors gives a negative result.
So, the first product is positive, because it has two negative factors
The second product is negative, because it has three negative factors.
The third product is special, because it has a zero factor, so the result is zero and we don't care about the other factors.
The fourth product is positive because it has four negative factors.
