Why is the root file system mounted read-only during boot and remounted with write
permission later on?

A.
Because the disk has its own write protection that cannot change by the operating
system.

B.
To avoid other operating systems overwriting the Linux root partition

C.
Because this way crackers cannot collect information about root with boot sniffers

D.
Because if problems with the root file system are detected during the boot, fsck can be
run, without risk of damage.

E.
To avoid writing to the disk, unless the root password is known.