原则上来说，生物系统的衰老和死亡都不是必然的。

It is not, in principle, necessary for a biological system to age and die.

youdao

生物课上，学了生物系统的物质循环。

In the biological class, has studied the living system material circulation.

youdao

生物课上，学了生物系统的物质循环。

In the biological class, has studied the living system material circulation.

youdao