When asking these questions, it'd be helpful if you could post your best attempt at answering, along with specific questions about what you don't understand.
If we just give you the answers, it's unlikely to be very helpful.
The first sentence is establishing a contrast. The speaker tried to get to the center, but couldn't get there. Only one of the choices given fits with that meaning of contrast. Any of the other three would result in a nonsensical statement.
Question #2, I'm assuming, is meant to follow Question #1. Thus, it's providing the reason that the speaker wasn't able to get in. Again, only one of the choices given captures that meaning. The others would all seem to imply the opposite of this.
Care to take a try at answering it now?