Which of the following techniques is NOT ideal for immobilizing

Which of the following techniques is NOT ideal for immobilizing cell-free enzymes ?

1. physical entrapment by encapsulation
2. covalent chemical bonding to surface carriers
3. physical bonding by flocculation
4. covalent chemical bonding by cross-linking the precipitate

Covalent chemical bonding by cross-linking the precipitate is not ideal for immobilizing cell-free enzymes.