SC Guidance

Hello Everyone,

I am working on my verbal skills to do well in gmat. Little over a month ago when I gave GMAT I got 48Quant but 24 Verbal.
Thus for last few weeks I am working on improving my verbal skills. I have gone thru Manhattan SC couple of times with notes on sc by other users.

I have worked hard specially on SC but still feel that there is a lot more scope for improvement. Following issues i face in SC-

Q-1) In the Gulf of Mexico's 'dead zone,' every summer aquatic life is smothered in an area as large as the state of New Jersey, which, according to scientists, is caused by excess nitrogen flushed down the Mississippi River.

A) which, according to scientists, is caused by excess nitrogen flushed down the Mississippi River.
B) which scientists say is caused by excess nitrogen flushed down the Mississippi River.
C) the cause of which, according to scientists, is excess nitrogen flushed down the Mississippi River.
D) the cause of which is excess nitrogen flushed down the Mississippi River say the scientists.
E) that comes from the flushing of excess nitrogen down the Mississippi River, according to scientists.

A & B also seem right. I cant figure out specially in misplaced modifier which words have been moved??

Q-2) The reason I am hiring her is because her extensive experience in consulting has made her qualified for a seat on this important subcommittee.

A) because her extensive experience in consulting has made her qualified for
B) that her extensive experience in consulting have made her qualified for
C) that her extensive experience in consulting has made her qualified for
D) that her extensive experience in consulting has made her qualified to
E) because her extensive experience in consulting have made her qualified for

I mean I picked the wrong answer (A) where as the right one is (C). Cant figure out how to pick right one??

And another important problem, subject verb agreement

Q-3) The Food Marketing Institute (FMI) issued a report stating that an increase in time-starved consumers who lack the time and/or patience to do routine daily tasks such as shopping, have encouraged the growth of online grocery services.

A) have encouraged the growth of online grocery services.
B) have encouraged people to grow their usage of online grocers.
C) has encouraged online grocers to grow
D) has encouraged online grocers to be more patient.
E) has encouraged the growth of online grocery services.

Now I made a mess in verb agreement as I thought have encouraged is being pointed out to consumers so went for A which is wrong. How to find which verb is pointing to who??

Help will be appreciated,

Thanks,

Cheers,
GR