Researchers performing insert a weak electric current into the brain to stimulate it. A. Lesioning B. ESB C. EEG D. PET.

the answer would be c. sediment buildup on continental slope

                                                                                -hope this -

d. i assume the underlined word is "role." reformat if it isn't.

explanation:

role is closest to part because when replacing words, "playing a vital decision," "playing a vital activity," and "playing a vital dollar" don't make logical sense. the only meaningful choice is "playing a vital part."

this fits one of merriam-webster's definitions of role: "a function or part performed especially in a particular operation or process."