Please help on this one

Ascientific idea becomes valid when it is what?

18. over time, how do sand dunes tend to migrate? a. perpendicular to the movement of the wind b. in the same direction as the wind blows c. toward the wind d. in random directions

Alexa is preparing for a presentation by creating note cards identifying keywords that describe various features of each type of plate boundary. which words apply to all three of the convergent boundaries?