Here the basic question is one of deciding between ‘to keep from’
and ‘to keep it from’.
When we say that somebody does something to keep from being done, it might very well read that he does for himself rather than for any other. For e.g.
Tom hid the car to keep from being taken by the police; what is feared to be taken by the police? Is it the car or is it Tom? It is rather hazy. Here the lack of the object pronoun ‘it’ renders the plausibility that Tom hid to save himself. Similarly in the given text, the lack of the pronoun gives a twist that the chalice was probably buried by itself, rather a wrong premise. Hence for the sake of precision, the pronoun ‘it’ becomes necessary.
C is also to be rejected on the ground that the chalice was trying to avoid being stolen.
So, C, the precise answer without any distortion and verbosity is right.
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