Help with computer discussion homework

"Virtualization and Cloud Computing" Please respond to the following:

• Suggest how an organization should choose between Software as a Service (SaaS), Platform as a Service (PaaS), and Infrastructure as a Service (IaaS). Support your recommendation with an explanation.
• Salesforce.com, Facebook, Gmail, Windows Azure, Apple iTunes, and LinkedIn are examples of applications in the cloud computing space. Examine the benefits and possible problems for these organizations when it comes to cloud computing. Propose solutions to these problems. Predict the impact to an organization’s TCO and ROI.