Gyro, an expert in lifting and emplacing equipment atop tall buildings, contracted in a signed writing to lift and emplace certain air-conditioning equipment atop Tower's building. An exculpatory clause in the contract provided that Gyro would not be liable for any physical damage to Tower's building occurring during installation of the air-conditioning equipment. There was also a clause providing for per diem damages if Gyro did not complete performance by a specified date and a clause providing that "time is of the essence." Another clause provided that any subsequent agreement for extra work under the contract must be in writing and signed by both parties. With ample time remaining under the contract for commencement and completion of his performance, Gyro notified Tower that he was selling his business to Copter, who was equally expert in lifting and emplacing equipment atop tall buildings, and that Copter had agreed to "take over the Gyro-Tower contract." If Tower refuses to accept Copter's services, which of the following clauses in the GyroTower contract will best support Tower's contention that Gyro's duties under the contract were not delegable without Tower's consent? 
The exculpatory clause. The liquidated-damage clause. The "time is of the essence" clause.  The extra-work clause.