Which of the following should be the FIRST sentence after the rearrangement?

The sentences can be arranged in the following way to make a meaningful paragraph:
(The index number has been left unchanged for ease of understanding.)
E. Companies that are marketing a product face different challenges compared to those that are promoting a service. (This will form firsrt statement as it introduces a theme of differences in product and services marketing.)
A. If you are transitioning from products to services or vice-versa, you have to know and understand these differences to effectively promote and sell. (This will form second statement as the premise is further elaborated here.)
C. Understanding the different changes in product and service marketing can help you establish the right approach for this transition. (This would form as thirtd sentence. This sentence talks about the transitions whose premise has been set up in the last sentences.)
B. For example, a product is tangible, which means the customer can touch and see the product before deciding to make a purchase and a service is intangible.(This sentence elaborates by giving an example.)
D. Unless you understand the basic difference of tangibility, it will be a challenge to promote and sell your product. (This would form the last statement as it expounds on the 'tangibility' concept introduced in the previous sentence.)

E forms the first sentence after rearrangement.
Option E is correct.