Economic models
a. are useful because they do not omit any real-world details.
b. are usually composed of diagrams and equations.
c. are usually plastic representations of the economy.
d. are not useful because they omit many real-world details.

The answer is:

B. They must establish a fair labor market.

D. They must open up trade to other countries

In mixed-market economy, both government and the private sectors play equally important roles in managing/regulating the economy.

Establishing fair labor market is being done in order to prevent the private sectors from abusing their workers. Opening trade to other countries is being done by the government sectors through mutually beneficial foreign policies which will improve our total exports.