Their son's illness, once regarded not so serious

[Volcano1985]

Their son's illness, once regarded not so serious,.......... day by day so they .......... him to the nearest hospital immediately.

A) worsened / have taken
B) was worsening / took
C) worsens / take
D) has worsened / took
E) is worsening / have taken

The key is E, why not B?

[Mister Micawber]

B seems fine to me, too. Actually, there is nothing grammatically wrong with A, and C could be used in a narrative... and D also fits possible contexts.

A VERY poorly written question.
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Native English teacher at Mister Micawber's

[Volcano1985]

Thank you Mister