Geography

The Sargasso Sea is known as an oceanic desert because it has very low nutrient levels and minimal plankton, resulting in low biological productivity compared to other ocean regions.

Authoritative sources including the National Snow and Ice Data Center and scientific literature confirm that glaciers exist in approximately 50 to 51 countries worldwide. These include countries with glaciers in polar regions, high mountain ranges, and some oceanic islands. The exact number can vary slightly depending on definitions and data sources, but 51 is a commonly cited and accurate estimate. Therefore, option B (51) is correct.

The boundary line between Canada and the United States is primarily defined by the 49th Parallel North, which serves as the border from the Lake of the Woods to the Rocky Mountains.

The Gulf of Sidra is a body of water located on the northern coast of Libya, making Libya the correct answer. The other options are countries not bordering the Gulf of Sidra.

China is located to the north of Pakistan, sharing a border in the northern region of Pakistan.

The Gobi Desert is primarily located in northern China and southern Mongolia, making Mongolia the correct answer among the options provided.

Lesotho is a country completely surrounded by South Africa, making it an enclave within another country. The other options are not enclaved countries.

The Mekong River is known as the Lancang River in China. This is the commonly accepted name for the upper reaches of the Mekong within Chinese territory.

An isthmus is a narrow strip of land that connects two larger land areas, often with water on either side. This matches option D exactly.

There are exactly two landlocked countries in South America: Bolivia and Paraguay. This makes option A the correct answer.