"As" can mean: at the same time as
For example: As I was walking on the street, a car hit a pole.
-or-
"As" can mean: because of (reason)
for example: As it's raining, I'll use an umbrella.
This is more British English.
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As I was walking, I saw a grocery store selling Christmas trees.
As I live in an apartment, I won't buy one.
