Given a scientific passage and an answer, generate a question for the given answer.

Passage: Lasers Most light emitted by atoms is polychromaticcontaining more than one wavelength. In contrast, lasers (from light amplification by stimulated emission of radiation) emitmonochromatic light a single wavelength only. Lasers have many applications in fiber-optic telecommunications, the reading and recording of compact discs (CDs) and digital video discs (DVDs), steel cutting, and supermarket checkout scanners. Laser beams are generated by the same general phenomenon that gives rise to emission spectra, with one difference: only a single excited state is produced, which in principle results in only a single frequency of emitted light. In practice, however, inexpensive commercial lasers actually emit light with a very narrow range of wavelengths. How a CD player uses a laser to read a CD. Inside a CD is a flat, light-reflecting layer called "land. " On the land are many "pits" recorded in a spiral-shaped track. (From the laser's point of view, pits are actually the "bumps" shown here because the master disc with pits is duplicated negatively, turning the pits into bumps. ) Pits have the same light-reflecting surface as land, but there are differences in the frequencies of the reflected light in the pit and the land, making light reflected by pits relatively dark compared with light reflected by land. Answer: wavelength
Light emitted by atoms is polychromaticcontaining more than one of these?

Passage: Imagine walking in the forest in Figure below . The towering trees have been growing here for hundreds of years. It may seem as though the forest has been there forever. But no ecosystem is truly static. The numbers and types of species in most ecosystems change to some degree through time. This is called ecological succession. Important cases of ecological succession are primary succession and secondary succession. Answer: ecological succession
The numbers and types of species in most ecosystems change to some degree through time and this is called?

Passage: Rounding is done when one or more ending digits are dropped to get the correct number of significant figures. Simple rules state when to round up and when to round down. Answer: rounding
What is it called when one or more ending digits are dropped to get the correct number of significant figures?