1. first; second; third;
fifth; eighth; ninth
2. tenth; twelfth; eighteenth
3. twentieth; thirtieth; fortieth;
fiftieth; eightieth; ninetieth
请写出下列序数词，注意它们的构成规律。 1. 第一_______ 第二_
1. first; second; third;
fifth; eighth; ninth
2. tenth; twelfth; eighteenth
3. twentieth; thirtieth; fortieth;
fiftieth; eightieth; ninetieth