Because is the correct answer. The word because is connecting the two statements together and explaining how they finished their work.
Answer:
A medicinal concoction or mixture of more than one substance.
Explanation:
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Don Anselmo's action allows him to continue to plant trees...
It is dark because Barnardo has trouble seeing the other guard.
This scene takes place at midnight and it is so dark Barnardo cannot see the other guard. Barnardo shows up for his shift, but he can't clearly make out who Francisco is. Francisco has the same problem because instead of answering Barnardo he tells him to identify himself first. They get it figured out and Barnardo begins his shift.