instruction:
This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.
question:
Transport in the Republic of the Congo includes land, air and water transportation. The country's rail system was built by forced laborers during the 1930s and largely remains in operation. There are also over 1000 km of paved roads and two major international airports (Maya-Maya Airport and Pointe Noire Airport) which have flights to Paris and many African cities. The country also has a large port on the Atlantic Ocean at Pointe-Noire and others along the Congo River at Brazzaville and Impfondo.
answer:
What forms of transport are available in the Congo?


question:
The emergence of Susa as a city, as determined by radiocarbon dating, dates back to early 4,395 BC. There are dozens of prehistoric sites across the Iranian plateau, pointing to the existence of ancient cultures and urban settlements in the 4th millennium BC. During the Bronze Age, Iran was home to several civilizations including Elam, Jiroft, and Zayande River. Elam, the most prominent of these civilizations, developed in the southwest of Iran, alongside those in Mesopotamia. The emergence of writing in Elam was paralleled to Sumer, and the Elamite cuneiform was developed since the 3rd millennium BC.
answer:
What ancient city dates back as early as 4395 BC in Iran?


question:
The quality of a vacuum is indicated by the amount of matter remaining in the system, so that a high quality vacuum is one with very little matter left in it. Vacuum is primarily measured by its absolute pressure, but a complete characterization requires further parameters, such as temperature and chemical composition. One of the most important parameters is the mean free path (MFP) of residual gases, which indicates the average distance that molecules will travel between collisions with each other. As the gas density decreases, the MFP increases, and when the MFP is longer than the chamber, pump, spacecraft, or other objects present, the continuum assumptions of fluid mechanics do not apply. This vacuum state is called high vacuum, and the study of fluid flows in this regime is called particle gas dynamics. The MFP of air at atmospheric pressure is very short, 70 nm, but at 100 mPa (~6997100000000000000♠1×10−3 Torr) the MFP of room temperature air is roughly 100 mm, which is on the order of everyday objects such as vacuum tubes. The Crookes radiometer turns when the MFP is larger than the size of the vanes.
answer:

What indicated the quality of a vacuum?