________ he arrived, we started reading.

A. Once
B. As soon as
C. While
D. Till

Once he arrived, we started reading.

Usage of ONCE and AS SOON AS

“Once” and “as soon as” have the same basic meaning as “after”, but they emphasize the 2nd action or event starts immediately after the 1st action or event finishes. We can say that “once” and “as soon as” have the same meaning as “immediately after”. I fell asleep immediately after I got in my bed.
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