Answer:
You can use 5 to simplify 5 and 15, giving you 1 and 3
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A)

B)

C)

D)

You were correct, the exact answer is A. The others do not equal 23 besides that one. Good job!
Answer:
see below
Step-by-step explanation:
Scientific notation is a * 10 ^b
a must be a number between 1 ( including 1 ) and less than 10
12 is greater than 10 so it is not scientific notation