MPS = ΔYΔS
Since change in savings ΔS is always non-negative relative to change in income ΔY, MPS can never be negative.
APC = YC
At the break-even point, consumption equals income, so:
APC=YC=YY=1
Therefore, APC can have a value of 1.
MPS can never be negative while APC can have value as 1.