老师您好！

看到书上这样一段话：

These synthetic plastics take several hundred years to biodegrade, which can be bad for the environment.

If people throw plastic items on the ground or into rivers, they can end up in the sea.

(1) to biodegrade 是悬空不定式吗？意味着to biodegrade 是及物动词？

(b) 为什么ground前要加the, rivers不加the, 而sea前又加了the呢？

感谢老师！