The detail about the systems of improving memory are not popularized because:

The detail about the systems of improving memory are not popularized because:

a

( ) Their designers have financial interests over their students.

b

( ) The few people who know such detail are never seriously questioned about them.

c

( ) There are so many systems that their designer keep them secret to avoid competition.

d

( ) There has always been a lack of interest in them.

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