GMAT Sentence Correction (SC) | Life is possible on earth because earth can endure lethal levels

Hi,

This question is from e-gmat
Sentence: "Life is possible on earth because earth can endure lethal levels of gamma radiation because of the presence of ozone layerthat has been blocking the harmful radiation and keeps the surface of the earth conducive for life."
a. that has been blocking
b. that has blocked
c. that blocks
d. that blocked
e. having blocked

Correct ans is - c.that blocks

I have identified the correct answer. But my question is about the tenses of the options
a.
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