Bipartite Graphs and Matchings · Easy problem

In a small company, workers $1,2,3$ can be assigned to projects $a,b,c,d$ as follows: worker $1$ only to project $a$, worker $2$ only to project $a$, and worker $3$ to any of $a,b,c,d$. A manager wants a minimum set of workers-or-projects that together touch every possible (worker, project) pairing in this list, to decide whom to interview first. What is the size of this minimum set?