Which of the following commands will find the string foo in the file file1.txt, regardless of foo being in upper or lower case letters?

Which of the following commands will find the string foo in the file file1.txt, regardless of foo being in upper or lower case letters?

A.
grep -i foo | file1.txt

B.
grep -n filel.txt < foo

C.
grep -n foo file1.txt

D.
cat file1.txt > grep -n foo

E.
cat filel.txt | grep -i foo

Explanation:
-i ignore case