Most land vertebrates can't drink seawater because
a. their kidneys can't excrete concentrated salt.
b. less water would need to be excreted by the kidneys.
c. they don't have kidneys.
d. their cells would eventually swell.

The best answer is A.

Unlike marine animals such as fish, land vertebrates do not have mechanisms to excrete high salt content. Their excretory organs namely the kidneys are not designed to handle concentrated salt content.

 Therefore if high  salt  content is retained in the body, then osmotic imbalance in cells of land vertebrates would  occur with the cells losing water because they would be hypotonic to their immediate surrounding and eventually die.

Marine fish are able to maintain osmotic balance in the sea water because they have specialized cells in their gills that actively eliminate salt and these fishes do not absorb any salt from the water they drink.

increases the risk of extinction 

explanation:

the answer is that if the species suffers from the disease, there is nothing stronger than the others, so it is possible to remove wholeness. an example of this is hunger in irish potatoes, which has a deadly disease bearish part of all potato harvest masses. pretty much ruins the land of crops.