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選項 in rotation輪流；B 選項on installment分期付款； D 選項on deposit支付押金；C選項in advance提前。根據句意，提前預定30臺。所以這題應該選擇C。
