The underground economy refers to used product sellers who sell in secondary markets. It is thought to make up of the US gross national product.

The underground economy refers to used product sellers who sell in secondary markets. It is thought to make up 3-30% of the US gross national product.

Explanation:

Household production refers to goods and services people produce for themselves.

Underground economy refers to buying and selling of goods and services that are concealed from the government to avoid taxes or regulations or because the goods and services are illegal.

If you know how much the economy works for a brief time, it doesn't matter.

If one knows how goods and services are produced for a decade or further, it might be more important to omit domestic production and development in the underground economy.

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explanation:

ans.

heterothermic organisms include those organisms that can switch between homeothermic (having body temperature higher than environment) and poikilothermic (having variation in internal body temperature) strategies.

some migratory birds show heterothermy as they decrease their metabolic rate during cold nights to keep their body temperature low. drop in metabolic rate allows increase in production efficiency of birds that reduces the internal body temperature to make it equal to external cold temperature during cold nights.

thus, the correct answer is 'birds allow their temperature drop in order to increase their production efficiency.'