Choose the equation below whose axis of symmetry is x = 0.

a. y = x^2 + 2x

b. y = x^2 − 16x + 58

c. y = x^2 + 2

d. y = x^2 − 4x + 2

An energy drink company claims that its product increases students' memory levels. to support its claims, the company issues advertisements claiming that 8 out of 10 people (chosen randomly from across the country) who tried their product reported improved memory. the missing component in this study is a .