Question:
Which position did Dietmar Keller hold in May, 1990?

Context:
 Dietmar Keller holds the position of member of the German Bundestag from Dec, 1990 to Nov, 1994. 
 Dietmar Keller holds the position of minister from Jan, 1989 to Jan, 1990. 
 Dietmar Keller holds the position of member of the Volkskammer from Mar, 1990 to Oct, 1990.

Answer:
member of the Volkskammer