A welder's qualification begins from the date of welding of the test piece. The European standard allows a qualification certificate to remain valid for a period of two years, prov

A. the welding coordinator or other responsible person can confirm that the welder has been working within the initial range of qualification

B. working within the initial qualification range is confirmed every six months

C. both a and b

D. since they have passed the performance test, their certificate will be valid for 2 years and no more assessment is required