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Although custom prosthetic bone replacements produced through a new computer-aided design process will cost more than twice as much as ordinary replacements,custom replacements should still be cost-effective.Not only will surgery and recovery time be reduced, but custom replacements should last longer, thereby reducing the need for further hopital stays.

Answer is

C the degree to which the use of custom replacements is likely to reduce the need for repeat surgery when compared with the use of ordinary replacements.

to which...  是个定语从句修饰 the degree,  which 指代 THE DEGREE, 到达什么样的程度.

利用假肢达到的一种程度.