>>都说了ONLY by Moon, 为什么不能推出D选项的，基本不受其它的影响? 是因为题目那句话仅仅是针对Earth而言，所以不能说either Earth's or Mars's吗？还是有别的理解。谢谢～
>>但ONLY by 好像就可以理解为其它的没有影响吧？
Based on the statements, We know that:
A) the angle is kept stable because of the substential gravitational influence.
B) The larger the moon, the bigger gravitational influence. (The Moon is large enough to have major impact on Earch while the moons on Mar do not. All other small moons,if any, around Earth does not have substential gravitational influence. Hence Only the Moon has the major gravitational influence on the Earth.
These are the only facts we observe now. We don't know that Gravitational influences other than moons have little or no effect. For example, if some some day, Human beings can find something else on the planets or create some device which can generate sbustential gravitatinal influence, we can apply that to the Earth and see it tilts more than 23 degree. | 
