C179 Technical test

The Miss Yu's house is located near the center of the Nanhe City, it's rather crowded on the road, but luckily I left early, so I knocked on her door in the last two to three minutes before I was supposed to be late.

While I was waiting for the door to open, my mood was perturbed, because I had a guess, and it was this guess that caused my perturbation.

After waiting for around 10 seconds