单选题 Pure feldspar is a colorless, transparent mineral, but impurities {{U}}commonly{{/U}} make it opaque and colorful.
  • A. truly
  • B. rapidly
  • C. periodically
  • D. frequently
【正确答案】 D
【答案解析】[解析] commonly意思是“通常地”，与frequently(经常地，频繁地)意思相近。truly真实地，确实地；rapidly迅速地；periodically定期地。