GMAT Sentence Correction (SC) | The English physician Edward Jenner found that if experimental subject

The English physician Edward Jenner found that if experimental subjects were deliberately infected with cowpox,which caused only a mild illness, they are immune from smallpox.

A. which caused only a mild illness, they are immune from
B. causing only a mild illness, they become immune from
C. which causes only a mild illness, they are immune to
D. causing only a mild illness, they became immune from
E. which caused only a mild illness, they would become immune to

Its a Hypothetical case construction
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