What do experts believe is the most essential measure to be taken?

Read the following passage and mark the letter A, B, C, or D on your answer sheet to choose the word or phrase that best fits each of the numbered blanks from 25 to 29 

Sad Times in the Arctic

     The beautiful white bears found in the Arctic are in grave danger, and human beings are very (25) ________ the cause of this danger. As we burn fossil fuels to generate electricity or get us around town in our cars and trucks. We create a massive amount of carbon dioxide.  This gas has increased in (26) ________ so much over the past few centuries that it is causing global warming. 

     Polar bears need sea ice in order to live because this ice is their hunting (27) ________. The bears’ main source of food is the meat and blubber of seals. These animals come to the surface holes in the ice to breathe. It is there that the polar bears have (28) _________ to ambush the seals. Without any ice, there is nowhere for the bears to stalk their prey. The seals rarely venture onto land and spend their time out on the sea ice. 

     In recent years, the sea ice has disappeared in the Arctic during the late summer. This is dangerous for the polar bears, as they rely almost exclusively on seals for food. (29)  ________ brown and black bears, polar bears do not hibernate during the winter.

Mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the following questions. 

They’ll be able to walk across the river ______.

The philosopher _________ theories I am writing went insane before he died