Which of the following commands will reduce all consecutive spaces down to a single space?

Which of the following commands will reduce all consecutive spaces down to a single
space?

A.
tr -c ‘ ‘ < a.txt > b.txt

B.
tr -s ‘ ‘ < a.txt > b.txt

C.
tr -d ‘ ‘ < a.txt > b.txt

D.
tr ‘\s’ ‘ ‘ < a.txt > b.txt

E.
tr -r ‘ ‘ ‘\n’ < a.txt > b.txt