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[ketkoag]

Please explain the difference between B and E..
b is the correct answer but I selected E

[capnx]

B makes the casual relationship stronger.

By doing A vs Doing A

By doing A, X does B. So X accomplishes B by doing A; A is almost required for B to happen.
IE: By studying hard, you beat the GMAT.
So you beat the GMAT because you studied hard.

Doing A, X does B. "Doing" is a participle phrase modifying X (or X's action) but there no direct relationship to B.
IE: Studying hard, you beat the GMAT.
So you beat the GMAT while studying hard, but "studying hard" may not be the cause of you beating the GMAT.

[vishal1702]

B makes the casual relationship stronger.

By doing A vs Doing A

By doing A, X does B. So X accomplishes B by doing A; A is almost required for B to happen.
IE: By studying hard, you beat the GMAT.
So you beat the GMAT because you studied hard.

Doing A, X does B. "Doing" is a participle phrase modifying X (or X's action) but there no direct relationship to B.
IE: Studying hard, you beat the GMAT.
So you beat the GMAT while studying hard, but "studying hard" may not be the cause of you beating the GMAT.

well said capnx.....
i would also like to add that the uncertainty of the second part of the sentence makes By essential in the beginning of the sentence...by removing may, option E turns good enough:

Stimulating the immune system at the site of the influenza virus entry into the host, nasal vaccination helps to induce more effective and long lasting immunity in recipients.

Hope this helps....