In the following passage, some words have been deleted. Fill in the blanks with the help of the alternatives given. Select the most appropriate option for each blank.

Comprehension:
Many people believe that animals possess a sixth sense and know (1) ______ the earth is going to shake. Some experts (2) ______ that animals' more acute hearing helps (3) ______ to hear or feel the earth's vibrations. (4) ______ can sense an approaching disaster long before humans (5) ______ what's going on.