Consider a linear regression model estimating the fuel efficiency of a car in terms miles per gallon of gas (mpg) based on its origin (region A, B or C) and number of cylinders with the following formula:

mpg = b0 + b1*RegionB + b2*RegionC + b3*Cylinders

The estimated values of the regression coefficients are provided below:

b0 = 40.7

b1 = -0.91

b2 = 2.66

b3 = -3.15

Based on this model, if X is the mpg of a car with 4 cylinders originated from region B, and Z is the mpg of a car with 3 cylinders originated from region A, what is the value of X - Z:

A. -0.91

B. -3.15

C. -4.06

D. -6.72