When is it a good idea to hire subject matter experts from outside the organisation?

Question $10$Select more than one:

a$.$

Always $$ any project can benefit from expert help

b$.$

When the work is so specialised that it$$s not worth employing someone permanently to do it

c$.$

When outside experts can do the work significantly cheaper, quicker, and to a better quality standard than internal staff

d$.$

Whenever the project falls behind schedule