Carlos would like to apply for a job as an electrician at ABC Electrical Ltd, which is a unionized organization. As described within the collective agreement, in order for Carlos to be hired by ABC, he must already be a union member in good standing with the International Brotherhood of Electrical Workers Union (IBEW). If he is not already a current member of the IBEW, ABC would not be allowed to hire Carlos. This relates to the concept of the ________.
1) union shop
2) closed shop
3) modified union shop
4) open shop
5) agency shop