The contract of employment is the agreement made between the employer and the employee. This could be a written agreement or in an oral form. In addition, the contract will also include "custom and practice" agreements. These are how things are usually done in the workplace, for example, if the employer always gives the employees a day’s holiday in August. Even though this is not mentioned in the written contract, this will form part of the contract as it is the usual practice. A trade union may have negotiated(谈判达成)an agreement with an employer about conditions at work. The negotiated agreement will often form part of a contract.Some contracts will be illegal if: the employee gets all or part of their wages in cash; tax and national insurance contributions(国民保险税)are not paid; and the employee knows they are getting paid in this way to avoid paying national insurance and tax."Custom and practice" agreements are usually ______. A. included in the contractB. known to employees onlyC. always written clearly in the contractD. signed by both the employer and a trade union