You can become another user with su$.$ What the difference is between the two commands below?

su user

su $-$ user

a$.$ The command su $-$ is only used to change to root

b$.$ There is no difference, but for the dash to separate the parameter

c$.$ The command su $($without dash$)$ is only used to change to root

d$.$ The command su $-$ user reloads the environment variables and completely changes the shell in which the commands will be executed