Many people who call themselves teachers are not actually teachers because teaching is not their main source of income. Which of the following is assumed in the above discussion? ['Many people called teachers lack qualified professional knowledge and skills.', 'The amount of income can measure the degree to which a profession is valued by society.', 'Low income prevents teachers from dedicating themselves.', 'A person cannot be called a writer unless the writing is his main source of income. The same is true for teachers.']