题目选项分析

Narwhals can be called whales of the ice: in icy channels, ponds, and ice-shielded bays they seek sanctuary from killer whales, their chief predator, and their annual migrations follow the seasonal rhythm of advancing and retreating ice.

Narwhals can be called whales of the ice: in icy channels, ponds, and ice-shielded bays they seek sanctuary from killer whales, their chief predator, and their annual migrations which follow the seasonal rhythm of advancing and retreating ice.