A small local telephone company serves a total of 2,000 phone lines distributed among 1,800 customers. (Some customers have only one phone line while others have multiple phone lines.) Of these 2,000 phone lines, 1,500 are residential and 500 are business. The phone company has available to it the following lists: a list of the 2,000 phone lines; a list of the 1,800 customers; a list of the 1,500 residential phone lines; and a list of the 500 business phone lines. For each question, identify the sampling plan used to select the sample of phone lines.