Why is 1 + (−5) equal to −4? (1 point)

a) Because it is 5 units to the right of 1 on a horizontal number line
b) Because it is 5 units to the left of 0 on a horizontal number line
c) Because it is 5 units to the right of 0 on a horizontal number line
d) Because it is 5 units to the left of 1 on a horizontal number line