The following contracts cannot be specifically enforced, namely:—
(a) where a party to the contract has obtained substituted performance of contract in accordance with the provisions of section 20;
(b) a contract, the performance of which involves the performance of a continuous duty which the court cannot supervise;
(c) a contract which is so dependent on the personal qualifications of the parties that the court cannot enforce specific performance of its material terms; and
(d) a contract which is in its nature determinable.