Which of the following commands overwrites the bootload…

Which of the following commands overwrites the bootloader located on /dev/sda without overwriting the
partition table or any data following it?

A.
dd if=/dev/zero of=/dev/sda bs=512

B.
dd if=/dev/zero of=/dev/sda bs=512 count=1

C.
dd if=/dev/zero of=/dev/sda bs=440 count=1

D.
dd if=/dev/zero of=/dev/sda bs=440