Stored procedures differ from triggers in what essential way?

a$.$ Stored procedures are usually run intentionally by users, whereas triggers are generally run automatically by processes.

b$.$ Stored procedures are stored at the server, whereas triggers are generally stored at each user's client machine.

c$.$ Stored procedures may include some programming commands, whereas triggers are generally pure SQL$.$

d$.$ Stored procedures can change data. Triggers do not change data.