According to the passage, a designer of products is pre occupied with the following problems EXCEPT ______.

A．advise them on ways of giving a product customer appeal
B．accept responsibility for giving a product customer appeal
C．advise them on the best time to go ahead with production
D．consult them during the design and development stages

A．By making mistakes and having them pointed out by others.
B．By having their mistakes corrected.
C．By noticing their problems and making changes.
D．By listening to explanations from skilled people.