Carlos is using a number line to add another integer to –4. He begins by showing –4 on the number line, as shown below. Carlos adds the other integer on the number line and says that the sum is negative. A number line going from negative 5 to positive 5. The area between negative 4 to 0 is highlighted.

Which is true of the number Carlos added to –4?
a. The number must be a negative integer.
b. The number must be a positive integer.
c. The number could be 3 or any integer less than 3.
d. The number could be 4 or any integer greater than 4.