Answer:
However
Explanation:
In English, However can be used to join two independences sentence into one compound sentence. These independences sentence generally in contrast/oppose to one another.
1st. Sentence : James and his team didn't finish the project.
2nd. Sentence: They did work hard.
Working hard generally resulted in finishing the project. So, these two sentences basically in contrast with one another. Because of this, using 'however' to join the two sentences would be appropriate.
