Standing on top of the mountain we watched the sun the horizon.A. sink belowB. sinking

问题补充：

Standing on top of the mountain, we watched the sun the horizon.

A. sink below

B. sinking under

C. sinking below

D. sink under

答案：

A. sink below

B. sinking under

(答案→)C. sinking below

D. sink under

解析：短语辨析及非谓语动词。Sink below落到……下；sink down沉落，下沉。站在山顶上，我们看着太阳落到地平线以下。现在分词作宾补表示正在进行的伴随性动作，和 被修饰词是主谓关系，故选C。