Get your sources together dude...
The combine are an alien reace that even the Xen fear. The master race is the advisors (probably). The Civil Protection are modified humans and all combines from thereon are both modified humans and modified alien races (whose worl has also been conquered). The Striders are (possibly modified) alien animals.
The Xen have been hiding from the Combine for a long time, but then the Resonance Cascade came, Freeman caused havoc, and soon the Combine have been attracted by the cascade. The remaining Xen had been devastated and the few surviving Vortigaunts came to earth and now co-operate with the humans.
So um yeah the Quiz was great BTW!