Which of the following commands will send output from the program myapp to both stdout and the file file1.txt?

A.
cat < myapp | cat > file1.txt

B.
myapp 0>&1 | cat > file1.txt

C.
myapp | cat > file1.txt

D.
myapp | tee file1.txt

E.
tee myapp file1.txt

Explanation:
From the man pages:
tee – read from standard input and write to standard output and files