单选题
In parts of the Arctic, the land grades into the landfast ice so ______ that you can walk off the coast and not know you are over the hidden sea.
【正确答案】
B
【答案解析】(imperceptibly难以觉察地)。句意为：在北极的部分地区，陆地逐渐变成附着于陆地的冰，这一点难以觉察。你可以远离海岸，却不知道自己在隐秘的海洋之上。A． permanently永久地；C．irregularly不稳定地；D．precariously不牢靠地，不稳地。