Meter, kilogram and second are the SI units of length, mass and time respectively. They are internationally agreed standard units. Write a descriptive ,note about the history, methods of ination and definition of meter, kilogram and second

Which of the following conditions lead you to see an absorption line spectrum from a cloud of gas in interstellar space? the cloud is extremely hot. the cloud is visible primarily because it reflects light from nearby stars. the cloud is cool and lies between you and a hot star. the cloud is cool and very dense, so that you cannot see any objects that lie behind it.