You're a marketing analyst evaluating the return on investment (ROI) of three marketing campaigns for Fortune 500 companies:

• Campaign A: Conducted by Walmart, with an initial investment of $2 million and generated sales of $5 million.
• Campaign B: Conducted by Amazon, with an initial investment of $3.5 million and generated sales of $8 million.
• Campaign C: Conducted by Apple, with an initial investment of $1.8 million and generated sales of $4 million.

Requirements:

• Calculate the ROI for each marketing campaign.
• Analyze the effectiveness of each campaign in generating sales.
• Provide recommendations for optimizing future marketing investments for each company.