AlignedChoice
Aligned Answer Blanks
This PG code shows how to align answer blanks and restrict the type of student answer allowed.
PG problem file  Explanation 

DOCUMENT(); loadMacros( "PGstandard.pl", "PGunion.pl", "MathObjects.pl", "alignedChoice.pl", ); TEXT(beginproblem()); 
Initialization:
We need to include the macro file 
Context("Numeric"); $f1 = Formula("sin(pi/4)"); $f2 = Formula("sin(pi)"); Context()>functions>disable("All"); Context()>functions>enable("sqrt"); Context()>texStrings; $al = new_aligned_list(ans_rule_len=>10, numbered=>1, tex_spacing=>"3pt"); $al>qa( "\($f1\)", Real($f1>eval)>cmp(), "\($f2\)", Real($f2>eval)>cmp(), ); Context()>normalStrings; 
Setup: We disable all functions, and then enable square root. The aligned list is an array of pairs of questions and answers (with answer evaluators). We can add as many question and answer pairs to the list as we like. The options for aligned list are vertical alignment, horizontal alignment, cellspacing, tex spacing (which must include units of measure, such as "3pt"), whether or not the list is numbered, whether there is a column of equals signs, and the length of each answer rule. The defaults and other options are given below.

BEGIN_TEXT Find the sine of each of the following angles. $PAR \{ $al>print_q() \} $PAR Remark: Your answer cannot contain trigonometric functions. It must be an arithmetic expression or number. END_TEXT 
Main Text: It's a good idea to explain to students that certain types of answers are not allowed. 
$showPartialCorrectAnswers = 1; ANS( $al>correct_ans ); ENDDOCUMENT(); 
Answer Evaluation: As expected. 