China Set to Topple U. S. As Biggest Economy Sooner After Virus" Bloomberg (January 14, 2021). Such a comparison of the size of economies is based on.

the GDP.

Explanation:

The GDP (gross domestic product) is the sum of all goods and services produced within an economy, excluding the value of the goods and services used up in production.  There are three methods for measuring an economy's GDP.  They are the income method, the expenditure method, and the production(output) method.  The income method calculates the gross income for all the factors of production (rent, wages, interest, and profit).  The expenditure method calculates the GDP to be equal to consumption + government spending + Investment + net exports.  The production or output method uses the product value-added approach to compute the GDP.

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.'