Which of the following is true regarding to prima facie speed limit in the state of Texas?

a. The prima facie speed limit in the state of Texas is 70 miles per hour under all circumstances.
b. The prima facie speed limit in the state of Texas is 55 miles per hour on state and federal highways
c. The prima facie speed limit can be altered only because of a shortage of fuel, and if reduction of the prima facie speed limit will foster conservation purpose and safety.
d. The prima facie speed limits are determined to be the reasonable and prudent speeds under normal conditions; however, no person may operate a vehicle at a speed greater than is reasonable and prudent under circumstances that do in fact exist.
e. None of the above