Assigning R & S Configurational Prefixes

For 2-hydroxybutanal and 2,3-dibromobutanoic acid various configurational structures (A through L) are drawn below. In the six following questions you are asked to select those structures that satisfy the specified condition. Note that the numbering of the dibromoacid starts at the carboxylic acid carbon atom (#1). Enter letters (A through L), corresponding to your selections, in each answer box. If there is no structure that fits the description enter an X in the answer box.
Do not enter superfluous characters, since they will be counted as incorrect answers.

1.   Which hydroxybutanal structures have a R configuration? .......
2.   Which hydroxybutanal structures have a S configuration? ........
3.   Which dibromo structures have a 2-(R), 3-(R) configuration? ...
4.   Which dibromo structures have a 2-(R), 3-(S) configuration? ...
5.   Which dibromo structures have a 2-(S), 3-(S) configuration? ....
6.   Which dibromo structures have a 2-(S), 3-(R) configuration? ....

This script written by William Reusch, Dept. of Chemistry, Michigan State University. Please send comments and corrections to whreusch@pilot.msu.edu.