How is an emission spectrum of an element generated?

-when an electron in an atom is excited to a higher energy state.
-when an electron in an atom is in the excited state.
-when an electron in an atom returns from an excited state to a lower energy state.
-when an electron in an atom is in the ground state.

A+b→2c when the reaction begins, the researcher records that the rate of reaction is such that 1 mole of a is consumed per minute. after making changes to the reaction, the researcher notes that 2 moles of a are consumed per minute. what change could the researcher have made to effect this change?

Astudent reacts 13 moles of iron with 21 moles of oxygen according to the following equation:

Which are examples of how technology has advanced scientific understanding.1using hot water to sterilize medical equipment.2transplanting a human organ into another individual.3inserting genes from one sheep into another cell to make a cloneunderstanding the different structures that make up a cell.4examining microorganisms from the deepest parts of the ocean