Explain why the US economy is referred to as a mixed market. Answer in own words and don't make it long

According to the federal regulations, which of the following studies meets the definition of research with human subjects? a. a researcher uses the customs office’s passenger lists for ships bringing immigrants to the us between 1820-1845 to track the numbers of immigrants from certain ethnic groups. b. a researcher conducts a linguistic study of comments posted on a local public blog. c. a developmental psychologist videotapes interactions between groups of toddlers and their care givers to determine which intervention methods most effectively manage aggression. d. a researcher asks the director of a local free clinic about the number of patients in the last two years with newly diagnosed hiv/aids.