（-3x2y）•（13xy2），
=（-3）×13×x2•x•y•y2，
=-x2+1•y1+2，
=-x3y3．
去年“五一”小长假期间，某旅行社接待一日游和三日游的游客共1600人，收取旅游费共129万元，其中一日游每人收费150元，三日游每人收费1200元，问：该旅行社接待一日游和三日游的游客各多少人？
At the beginning of class, the raise of desks could be heard outside the classroom. [ ]
A. opened and closed
B. to be opened and closed
C. being opened and closed
D. to open and close
