What is the effect of external economies on trade and national welfare?

External economies can cause countries to be trapped in unwanted specialization patterns.
External economies can make international trade result in losses.
External economies always lead to a virtuous circle of production and low costs.
External economies are only beneficial for countries that are pioneers in a certain industry.
a) I and II are correct.
b) II and III are correct.
c) I and IV are correct.
d) II and IV are correct.