$1.$ About high$-$frequency $($transaction level$)$ data... answer all parts please

a$)$ High$-$frequency data tend to produce strongly negative correlations in price movements between transactions. What are the two reasons for that negative correlation?

b$)$ Market makers buy a share of stock at the bid price, and they sell it at the ask price. What is the name for the difference between the bid and ask prices? c$)$ Suppose there are three transactions in a $1-$minute interval. The returns for those three transactions are $0\mathrm{.}001,0\mathrm{.}02,$ and $0\mathrm{.}01.$ Calculate the realized volatility during that $1-$minute interval.