题目内容：
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 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 contract 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.单选题1.According to the first paragraph, a contract of employment _____________ .
A.must be a written one
B.could be in an oral form
C.should be signed by a trade union
D.ought to be a workplace agreement
2.“Custom and practice” agreement are usually ______________ .
A.included in the contract
B.known to employees only
C.always written clearly in the contract
D.signed by both the employer and a trade union
3.According to the passage, the trade union and employer may
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