Kaira's company recently switched to a new calendaring system provided by a vendor. Kaira and other users connect to the system, hosted at the vendor's site, using a web browser. Which service delivery model is Kaira's company using?

Platform as a Service $($PaaS$)$

Communications as a Service $($CaaS$)$

Software as a Service $($SaaS$)$

Infrastructure as a Service $($laaS$)$