The first question is about how to grow our economy. Basically, there are 4 ways to improve our economy.
1. Increase in the quantity of economic resources.
2. Improvement in the quality of eocnomic resources.
3. Advancement in techonology.
4. International trade.
Economic resources includes Natural Resources, Capital, Labour, Entrepreneurial skills.

The second question is to ask you to find a statement which can weaken these individuals' opinion and promote international trade. The basis of international trade is Comparative Advantage.
When a country has a lower Opportunity Cost to produce a specific product, then we can say this country has Comparative Advantage in producing the product over other countries.
Option D says this country's opportunity cost of producing Steel is high, so it is better off to engage in international trade to import steel from other countries.

The third question is also about Comparative Advantage and Opportunity Cost.
Now the song writer has two choices by spending the same amount time, either to write songs or to restore the farmhouse by himself.
Because he has professonal song writer, he can make more money from song writing. To restore the farmhouse by himself can save him money and provide him enjoyment. However, the money saved from restoration will be far less than the money he can make from song writer. This is because he has lower opportutity cost in song writing.

The answer for the last question is E. The supply of oats came down DRASTICALLY. That is to say, the decrease in supply is larger than the decrease in demand. As a result, the price goes up instead of going down.