An ironworkers union has been trying to negotiate a new contract with the contractor for months. The contract expired last night at midnight. All the workers showed up$,$ on time, this morning, then walked off the site. What kind of technique is being used?

a$)$ Lockout.

b$)$ Wildcat strike.

c$)$ Fact$-$finding to settle the dispute.

d$)$ Economic strike.

e$)$ Mediation.