---Where is Nick?--- He is standing in front of the window s_____ at the rain pouring down.

---Where is Nick?--- He is standing in front of the window s_____ at the rain pouring down.
---Where is Nick?--- He is standing in front of the window s_____ at the rain pouring down.

---Where is Nick?--- He is standing in front of the window s_____ at the rain pouring down.
没错,是填staring,现在分词短语当伴随状语,修饰standing

staring
stare at 聚精会神地看着。。。

stay

staring