The ls command is important for shell programmers, but it has some quirks. Offer solutions for the following:

a. ls gives you single-column output, but you want it to be shown in multiple columns.
b. ls shows output sorted alphabetically. Sort it by most recently modified to least instead.
c. Type two different ways to have ls include file size information.
d. The opposite of a: Force ls to output a one-column file listing.

a. ls -C
b. ls -t
c. ls -s
d.ls -l