The would be a because it makes more sense than the other sentences. If you read each of them a will make better sense.
A of course, Public speaking is nothing without a focused audience.
Some people are insecure, or downright untrustworthy of their surroundings. Usually, it's not your fault if they are fake towards you. They have usually adapted their fake selves on others that it becomes a normality.
Hope this helps!
I'm not quite sure why this is categorized under English, but I'll still help you. :)
When you're accustomed to a place, you've practically memorized every step you must take and don't even think about how to get there, places around the area, or back home from there. Even following a route you may not have necessarily explored before is simpler because you're familiar with the rest of the area. This is a psychological trick known as latent learning.
It's quite fascinating, and many of us take our subconscious mind for granted. Basically, you store useless information into the back of your mind that will be useful later on. You may not use it until that moment, but it is there.
