Assuming that the sums and products given below are defined, which of the following is not true for matrices?
1
A + B = B + A
2
AB = AC implies B = C
3
AB = 0 then always A = 0 or B = 0
4
\({\left( {{\rm{AB}}} \right)^{\rm{T}}}{\rm{\;}} = {\rm{\;}}{{\rm{B}}^{\rm{T}}}{{\rm{A}}^{\rm{T}}}\)