Answer: A) numbers of neutrons Explanation: Isotopes are variations of an element that have the same charge as the original element, but a different mass, which is caused by either more/less neutrons in the nucleus.

Acetylene is the only fuel gas suitable for gas welding because of its favourable flame characteristics of both high temperature and high propagation rates. Other fuel gases, such as propane, propylene or natural gas, produce insufficient heat input for welding but are used for cutting, torch brazing and soldering.