Twice the birds as last year will migrate to the

[BTGmoderatorDC]

Twice the birds as last year will migrate to the south this year.

A) Twice the birds as last year will migrate to the south this year.
B) This year, double the number of birds as last year will migrate to the south.
C) The birds that will migrate to the south this year will be twice that of last year's.
D) This year, twice as many birds will migrate to the south as did last year.
E) The number of birds that will migrate to the south this year will be twice that of last year's.

[GMATGuruNY]

Twice the birds as last year will migrate to the south this year.

A) Twice the birds as last year will migrate to the south this year.
B) This year, double the number of birds as last year will migrate to the south.
C) The birds that will migrate to the south this year will be twice that of last year's.
D) This year, twice as many birds will migrate to the south as did last year.
E) The number of birds that will migrate to the south this year will be twice that of last year's.

A: twice the birds as last year
Here, BIRDS are illogically compared to LAST YEAR.
Eliminate A.

B: double the number...as last year
Here, THE NUMBER is illogically compared to LAST YEAR.
Eliminate B.

C and E: that of last year's
Here that OF and last year's both serve to convey POSSESSION.
The result is an error of redundancy.
Eliminate C and E.

The correct answer is D.