“Ios: A Complete Version History”. 2018. Lifewire. https://www.lifewire.com/ios-versions-4147730.

Radcliffe, Brent. 2018. “Android Operating System”. Investopedia. https://www.investopedia.com/terms/a/android-operating-system.asp.

“What was Symbian OS?” Computer Hope  https://www.computerhope.com/jargon/s/symbian-os.htm

“Android Operating System” Techopedia. https://www.techopedia.com/definition/25106/android-operating-system

“Update Your BlackBerry 10 OS.” BlackBerry.  https://us.blackberry.com/software/smartphones/blackberry-10-os.

“What is BlackBerry OS? – Definition from Techopedia.” https://www.techopedia.com/definition/25196/blackberry-os.