In this problem we have that
the solution is the shaded area above the dashed line
so
Find the equation of the dashed line
x -----> outside temperature
y ----> inside temperature
Find the slope
take the points (0,20) and (10,30)
m=(30-20)/(10-0)
m=10/10
m=1
the equation is
y=x+20
therefore
the inside temperature is always 20 degrees warmer than the outside temperature
answer is option B