单选题
The______of medical knowledge are being pushed farther onwards as time goes on.
【正确答案】
C
【答案解析】解析：frontier n.(本义)边境，边界；边疆：(引申)(科学开拓的)新领域(beyond the frontier of knowledge；at the frontier of space exploration)。front n.前面，正面：前线，战线。border n.边界，边境；边缘，界线。edge n.边缘，棱；刀口。