分解因式：x2-y2-3x-3y．
原式=（x-y）（x+y）-3（x+y）
=（x+y）（x-y-3）．
Meeting my old classmates at the 66th anniversary celebration of our school was an unforgettable
moment, _____ I will always treasure. [ ]
A. that
B. one
C. it
D. what
怎样利用沼气肥？
