蚕丝是由极细的纤维组成的。

Silk is made up of very fine ___filaments.

灯泡里的灯丝断了，所以不亮了。

The light bulb doesn't work because its ___filaments are broken.

显微镜下可以看到蘑菇的菌丝呈现出细长的丝状结构。

Under the microscope, you can see the mushroom's ___filaments forming thin, thread-like structures.