题目

作文：描述徐悲鸿的骏马图，介绍中国水墨画的特点，说一说你是否喜欢这张图，并说明原因，不少于五句话。This is a picture by __________________________________________________._________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________在一定条件下，可发生反应：RO3n-+F2+2OH-==RO4-+2F-+H2O。则RO3n-中R元素的化合价是(    )       A．+4B．+5C．+6D．+7