Why Has the Austin City Council Had One Hispanic Member

Why has the Austin city council had one Hispanic member and one African-American member for the past few decades?

A) Such a system was mandated by a U.S. Supreme Court decision.
B) Such a system was mandated by a Texas Supreme Court decision.
C) The single-member district system encourages such an outcome.
D) The proportional representation system encourages such an outcome.
E) There has been an informal "gentleman's agreement" in place that reserves one seat for a Hispanic member and one seat for an African-American member.