Rome was not built in a day;
On the surface, the water is frozen by three feet, which can't be caused by cold weather.
Exactly, everything has its potential and long-standing factors, and it can't be formed suddenly. It is often used to talk about the formation and achievements of contradictions. Used in a derogatory sense.
The embankment of a thousand miles was destroyed in the ant nest;
A thousand miles long dike burst because of a small ant hole.
Used to describe a small matter, a little carelessness will lead to great disaster.
Self-understanding:
It doesn't seem terrible to freeze for three feet, but many days of cold will turn water into three feet of ice.
In the levee of a thousand miles, a small ant hole seems insignificant, but biting in the ant hole for a few days will make the long levee collapse.
It can be seen that the two have similarities in meaning, and both emphasize that small makes a big difference, and C should be chosen.
I hope I can help you ~