Please help me with these questions.

a) Companies may not pay dividends all the times. What could be possible reasons for a company not to give dividends to its shareholders?

b) In the book, we talk about Constant Dividend Model and Constant Growth Dividend Model. "If the growth in the latter model becomes zero, then the model becomes Constant Dividend Model." Do you agree with the statement or not?

c) If a company is not paying any dividend, then how you can evaluate the true value of the stock of the company.

d) Company AAA Inc. has preferred stocks and stocks,bonds, debentures, and callable bonds. Which of these instrument is least risky? which one is most risky? Order these securities in terms of riskiness.