An interrupted pilot of an automatic boiler:
a. Burns continuously, like that in a home furnace
b. Ignites before the main burner ignites, and is extinguished when the main burner is extinguished
c. Is one that has been extinguished by an inadequate flame signal
d. Ignites, is proven stable and extinguishes after the main flame is established
e. Must be manually ignited
Answer: D