It's more important to what a person does because as they say, actions speak louder than words do. A person can think a certain way but not act on it but what a person does reflects what they decide internally and what they wanted to do with that idea externally showing that they are serious with that idea of theirs in their head.
B, “They saw the concert in the park last weekend.”
A concert in the park can be considered by some a “full” noun, or have a be a modifier, but both are correct.
Answer:
A. Someone surely forgot to lock the door yesterday.
B. They said that they decided to spend their holiday in Spain.
C. It is very hard to understand the instructions which are written in the manual.
Why did the chicken cross the road