单选题 Dermatologists say that it is better to wear sun block ______ than not to wear it at all.
【正确答案】 A
【答案解析】[解析] 词汇：副词
1．这是一道词汇题，选项均为副词。
2．本题需要选择一个副词，修饰动词wear。periodically意为“定期地”，A为正确选项。