While we welcome large orders, we must ask that you place orders of 30 units or more ( )
A. in rotation
B. on installment
C. in advance
D. on deposit
答案：C 【解析】A选项 in rotation轮流；B 选项on installment分期付款； D 选项on deposit支付押金；C选项in advance提前。根据句意，提前预定30台。所以这题应该选择C。
