Is unwillingness to be appointed same as resignation?

Hi, 

FACTS: An auditor was appointed for 5 years on 30/09/2014 for FY2014-15 till FY2019-20. After certifying the accounts & providing the audit report on 01/09/2015 for FY15, he expressed his unwillingess to be reappointed as auditor for FY2015-16.

QUERIES:

a) Does this amount to resignation by the auditor?

b) Is this casual vacancy caused by resignation?

c) Is the auditor required to file ADT3?

d) If the auditor sent his letter expressing unwillingness on 01/09/2015, can he sent a formal resingation letter on 08/09/2015?