GMAT critical reasoning diagnostic test

In 1990 all of the people who applied for a job at Evco also applied for a job at Radeco, and Evco and Radeco each offered jobs to half of these applicants. Therefore, every one of these applicants must have been offered a job in 1990.

The argument above is based on which of the following assumptions about these job applicants?

Recent audits revealed that Banque Card, a credit service, has erred in calculating the interest it charges its clients. But Banque Card’s chief accountant reasoned that the profits that the company shows would remain unaffected by a revision of its clients’ credit statements to correct its previous billing errors, since just as many clients had been overcharged as undercharged.

Which of the following is a reasoning error that the accountant makes in concluding that correcting its clients’ statements would leave Banque Card’s profits unaffected?

The number of musicians employed to play accompaniment for radio and television commercials has sharply decreased over the past ten years. This has occurred even though the number of commercials produced each year has not significantly changed for the last ten years.

Which of the following, if it occurred during the past ten years, would contribute LEAST to an explanation of the facts above?

Consumer income reports produced by the government distinguish between household and families by means of the following definition: “A family is a household containing a householder and at least one person related to the householder.” Except for the homeless and people in group living quarters, most people live in group living quarters, most people live in households.

According to the definition above, which of the following must be true?

The country of Maravia has severe air pollution, 80 percent of which is caused by the exhaust fumes of cars. In order to reduce the number of cars on the road, the government is raising taxes on the road, the government is raising taxes on the cost of buying and running a car by 20 percent. This tax increase, therefore, will scientifically reduce air pollution in Maravia

Which of the following, if true, most seriously weakens the argument above?