Which innovative retail strategy did Texas Instruments implement in the 1990s that helped the TI graphing calculators become the standard in mathematics education?

Texas Instruments calculators have dominated the educational mathematics market for decades. The TI-84 series, despite minimal hardware updates, continues to maintain premium pricing and market dominance in classrooms worldwide. This poll explores a fascinating aspect of TI's retail strategy that helped cement their position in the mathematics education industry.

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Creating teacher training programs and providing free calculators to educators who would then recommend them to students
Designing calculators specifically to align with the College Board's requirements for the SAT and AP exams
Implementing a subscription model where students paid a small monthly fee rather than purchasing the calculator outright
Pioneering the educational technology buyback program where schools could trade in old calculators for discounts on newer models