The following table shows the number of clients who attended a particular accountancy practice over the last four weeks and the total costs incurred during each of the weeks:
Applying the high low method to the above information, which of the following could be used to forecast total cost ($) from the number of clients expected to attend (where x = the expected number of clients)

A．A standard which is kept unchanged over a period of time
B．A standard which is based on current price levels
C．A standard set at an ideal level, which makes no allowance for normal losses, waste and machine downtime
D．A standard which assumes an efficient level of operation, but which includes allowances for factors such as normal loss, waste and machine downtime

A．(iii) only
B．(i), (ii) and (iii) only
C．(i), (ii) and (iv) only
D．All of the above