A staffing firm provides landscaping services for clients on an ongoing basis. The staffing firm selects and pays the workers, provides health insurance, and withholds taxes. The firm provides the equipment and supplies necessary to do the work. It also supervises the workers on the clients' premises. Client A reserves the right to direct the staffing-firm workers to perform particular tasks at particular times or in a specified manner, although it does not generally exercise that authority. Client A evaluates the quality of the workers' performance and regularly reports its findings to the firm. It can require the firm to remove a worker from the job assignment if it is dissatisfied. Who is the employer of the workers?