What is the difference between Hard link & Soft link?

[a] Hard links cannot links directories ( cannot link /tmp with /home/you/tmp)

[b] Hard links cannot cross file system boundaries ( cannot link /tmp mounted on/tmp to 2nd hard disk mounted on /harddisk2)

[c] Symbolic links refer to a symbolic path indicating the abstract location of another file

[d] Hard links, refer to the specific location of physical data.