To find the inverse of a matrix by hand you

A. always change the sign of the two opposite elements and switch the place of the other two.

B. choose a variable matrix of the same size, multiply by the original matrix and set it equal to the identity matrix, then solve the linear system that that created.

C. fill up a matrix of the same size with all zeroes and then put ones on the main diagonal.

D. just pray for inspiration to come up with the matrix.