Read the following context and choose the best option to answer the question.
Context: My youngest brother had a surprise waiting for me when I went next door after work this afternoon . He came out with what looked like a book at first and I asked him if it was for me . Well , it was for me and it 's ' The Last Unicorn ' movie on DVD ! Was I excited and my jaw dropped open .
Question: What may be the reason for their jaw dropping open ?
Options: 
- Because they were bored .
 - Because they were thirsty .
 - Because they were so surprised .
 - Because they were hungry .


Because they were so surprised .