An industrial design engineer is developing a magnetic sorting system to separate ferrous contaminants from grain. The engineer constructs four prototype electromagnets, each consisting of a solenoid wound around a central core. Each prototype carries an identical constant electrical current:

• Prototype A: $120$ turns, wooden core
• Prototype B: $120$ turns, soft iron core
• Prototype C: $450$ turns, wooden core
• Prototype D: $450$ turns, soft iron core