单选题
Although Venus is the nearest planet of the Earth, little is known about it because it is ______ covered by thick clouds.
【正确答案】
D
【答案解析】[解析] 副词词义辨析。closely“接近地”；curiously“好奇地”；regularly“有规律地，整齐地”；constantly“经常地；坚持不懈地”。根据句意“Venus常年被厚厚的云覆盖”可知答案为D。