Each of the four fundamental forces (gravity, electromagnetism, and the strong and weak nuclear forces) has a kind of theoretical knob.
四种基本力（万有引力、电磁力、强核力和弱核力）中的每一种都有一个理论上的旋钮。
2
The little bamboo tree had been growing underground for the past four years, in the fifth year and beyond, it developed a root system strong enough for its ground growth.
过去的四年里，这株小竹子一直在地下生长，第五年开始，它形成了足够强大的根系，可以在地面生长。
3
Together the four models provide strong coverage in a number of ways.