Some solid evidence proves that water really exists on Mars, but unfortunately, there is still no of life.
A. mark
B. sign
C. symbol
D. signal
网友回答
A. mark
(答案→)B. sign
C. symbol
D. signal
解析：考查名词。A标记；B迹象；C象征；D信号。句意：有确凿的证据证明火星上存在水，但不幸的是，仍然没有生命的迹象。故B正确。