Step 1 -- Answers to the grammar exercises: (一) 1-5 ADCBC 6-10 DBBCB 11-15 ACACC 16-20 BDBCD 21-25 ABCAB (二) 1-5 DAABB 6-10 CDCAB 11-15 DADCB 16-20 DBDBC 21-25 DDABD