【正确答案】 D

【答案解析】[解析] 本题考查近义副词的辨析。A选项unceasingly和B选项continuously意思都为“不断地，不停地，without stopping”，例如：The telephone rang continuously for two minutes. 电话铃声连续响了两分钟。C选项gradually意为“渐渐地，逐渐地”，例如：The climate gradually becomes warmer.气候逐渐变暖。D选项continually意为“不断地，频繁地，repeated many times”，例如：The telephone has been ringing continually all morning.整个早晨电话铃声不断。此选项与constantly意思相近，符合本句意思。