桥拱形跨越河流。

The bridge ___arched over the river.

突然一道闪光拱形划过天空，随后传来回响的隆隆声。

Suddenly a flash ___arched across the sky, followed by an echoing rumble.

猫看到狗时拱起了背。

The cat ___arched its back when it saw the dog.