Pls help me to choose a correct answer.

Psychologists believe that taking risks has always
been part of human nature. For early humans,
risk-taking (1) must / can’t have been part of
everyday life. Psychologists think that early human
risk-takers (2) may / can’t have been more likely to
explore new places, possibly f nding a new source of
water or food. Such individuals (3) can’t / might also
have risked doing things differently, such as using
a new kind of weapon or animal trap. These acts
(4) must / mustn’t have given the risk-taker a great
sense of achievement, but (5) can’t / could also have
profoundly benef ted his or her group by improving
their lives in some way.