Which set of frequencies would yield the largest beat frequency

1
Which set of frequencies would yield the largest
beat frequency?
(green) 20 Hz and 21 Hz (red) 200 Hz and 210
Hz (orange) 2000 Hz and 2100 Hz (pink) 20000 Hz
and 21000 Hz
2
Which string has the most number of normal modes
below 20000 Hz (limit of hearing)?
(green) L 1.0 m, v 300 m/s (red) L 0.5 m, v
150 m/s (pink) L 2.0 m, v 300 m/s (orange)
L 0.5 m, v 300 m/s
3
Consider two pipes of the same length. A is
closed at one end and B is open on both ends.
Which of the following is the highest frequency?
(green) The first overtone of A (pink) the
fundamental of A (red) the first overtone of
B (orange) the fundamental of B