When the technique known as gene-splicing was invented in the early 1970's, it was feared that scientist might inadvertently creat an"Andromeda strain",a microbe never before seen on Earth that escape from the labortatory and (it would kill vast numbers of humans who would have no natural defense against it)
c:kill vast numbers of humans who would have no natural defense against it.
d:kill vast numbers of humans who have no natural defense against it.
c is correct,But I don't know the exact reason for the choice.
