As far as control tests are concerned, which of the following expressions is correct?

a. The auditor uses both planned detection risk and tests of controls to determine the scope of the audit tests that he is to perform.

b. The higher the control risk, the less evidence that has to be collected to demonstrate the effectiveness of internal controls.

c. When auditors expect to use evidence that was collected in prior periods and has already demonstrated the effectiveness of certain internal controls, auditing standards require that tests of controls be done every two years.

d. All of the above.