Against Spam : new step to register

Idea against spam :
a new step to register,
should answer to a random question from a pool of simple questions,
like this one :
What is the short for kilohertz ? (should reply : kHz exactly)
a new step to register,
should answer to a random question from a pool of simple questions,
like this one :
What is the short for kilohertz ? (should reply : kHz exactly)