Efofex Parameters have a no repeat feature that can greatly simplify the creation of questions.
Turning No Repeats on prevents (if possible) two parameters from being given the same value. For example:
$a=prime(1,100)
$b=prime(1,100)
$c=$a*$b
Both $a and $b will be selected from primes between 1 and 100 and $c will always have two prime factors - unless $a and $b happen to be the same prime in which case $c will be a square number. This is almost certainly NOT what you want to occur! Turning on No Repeats will prevent the case of $c being a square number which greatly increases consistency in your question.