(Refer to CAP 698 figure 4.23) What is the reason for the change in temperature lines at 17000ft?
A) It is the height below which engine relighting is permitted
B) It is a correction for the fact that the manual uses JET ISA rather than Standard ISA
C) It is the altitude below which it is not permitted to use A/C set to HIGH
D) It is the altitude above which the temperature remains constant

Even tho there is not a set rule for answer A. Im hesitant between A and C.

Can someone who knows explain this question to me please.