We used to work in the same office and we _____ have coffee together after work. [ ]

I wonder how he could ______ not handing in his math exercises yesterday, for his math teacher is always strict with his students.

What bad weather yesterday!—Yes. It _______ a few trafficaccidents.

We cannot figure out _______ quite a number of insects, birds, and animals are dying out.

All of the following are the themes of The Call of the Wild, EXCEPT _____.