Which of the Following Is True of the Herfindahl-Hirschman Index

Which of the following is true of the Herfindahl-Hirschman Index (HHI) ?

A) The HHI is used to determine whether the Justice Department will challenge a vertical merger.
B) The HHI is calculated by multiplying the market shares of the two firms who intend to merge.
C) The smaller the HHI, the less concentrated the market, and the less likely the Justice Department is to challenge the merger.
D) A postmerger HHI below 1,000 is likely to be challenged.