提建议：We should / shouldn't... — 英语选择题

Lily 看见公园里有游客随手扔垃圾，还想摘花。她想给公园写一条环保建议（advice）。下面哪一句是正确且合理的建议？A. We shouldn't keep the park clean. B. We should throw rubbish on the grass. C. We shouldn't pick the flowers in the park. D. We should pick the flowers and keep it green.