15 !
generally, when going down a group on the periodic table:
a. atomic radii decrease
b. ionic radii increase
c. electronegativity increases
d. reactivity decreases

Observation and experimentation have led many scientists to accept a theory about the origin of the universe. this theory is called the big bang theory. scientific evidence collected and observed by scientists around the world suggests that the universe is ever expanding from a hot and dense initial state. what makes this a scientific theory? (2 points)