Why do Texas school districts typically adopt textbooks pre$-$approved by the state?

a$.$ Adopting non$-$approved textbooks is often too costly an endeavor.

b$.$ The state's experts typically approve the most thorough textbooks anyhow.

c$.$ The amended state constitution requires school districts to do so$,$ except in "novel" academic subjects.

d$.$ Very few textbook publishers are willing to meet Texas's stringent educational standards.