Columns 3 and 4 each contain two blue highlighted cells which contain candidates of value 7. In fact, they are the only cells in those
columns which do contain candidate 7. Consequently, those 7s must provide the solution for 7 within Columns 3 and 4. In so doing, they will
also provide the solution for 7 within rows B and D. That being the case, any other 7s in rows B and D cannot be solutions. Four such
candidates exist, and are highlighted in magenta. They can be safely removed.