Answer:
a. A + C < B + C
Explanation:
when we know that A < B, then adding the same amount to both sides did not change the relationship between both sides.
this is like having a balance with 2 cups. one side is heavier than the other, so the heavier cup is down.
if we add the same weight to both cups, the situation will not change.
that is why a. is the right answer.
b. would only be right, if C is negative.
for positive C the same argument as for A applies. a smaller amount (or weight) stays smaller also after multiplying both sides by the same number.
but because this option would only be right for a subset of the possible values, this is not true in general.
c.
this is not true at all.
if we multiply the expression by -1, then the inequality sign has to flip. < becomes >, > becomes <.
which did not happen here.