Which of the following statements represents related$-$constrained diversification?

a

A firm earning more than $95$ percent of the revenues from a single line of business

b

A firm that earns more than $70$ percent of its revenue from its main line of business and the rest from businesses located along the value chain

c

A firm that earns less than $70$ percent of its revenue from its main line of business and whose other lines of business share product, technological, and distribution linkages with the main business

d

A firm that earns more than $70$ percent of its revenue from its main line of business and the remainder from other lines across different value chains