保持清洁相关祈使句 — 英语选择题

教室大扫除后，班长想提醒大家保持教室清洁。下列句子中，哪一项是正确且最得体的表达？ A. Not litter on the floor. Keep it clean! B. Don't litter on the floor. Keep it clean! C. Doesn't litter on the floor. Keeping it clean! D. Don't litters on the floor. Keep it cleanly!