試題詳解

【已刪除】45. According to the passage, modern techniques for dental fillings were developed _____.
(A) when bacteria-related infections were scientifically confirmed and proved
(B) when the dentists could use only one hand while operating a dental drill
(C) when laughing gas was no longer used for inhalation analgesia and anesthesia
(D) when they were based upon biological principles and microscopic evaluation