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 ~