18 患者屈光不正處方為+3.00D/-3.50DC×180,而鏡片前表面屈光力為+5.00 D,則鏡片的後表面屈光力為何? (A)-5.50D/+3.50DC×180 (B)-2.00D/-3.50DC×090 (C)-5.50D/+3.50DC×090 (D)-2.00D/+3.50DC×090