You are given a sentence and a question. You're expected to write the correct answer based on the sentence.

Sentence: A jackfruit experiences low friction on a red surface and high friction on a purple surface. Question: Which surface is smoother?
red surface

Sentence: A valley is significantly rougher than a mountainside. Question: This means that lava rolling down which surface will reach a greater top speed?
valley

Sentence: When playing pinball on two different machines, joe noticed that the pinball moved quickly over the glass surface whereas it barely moved at all over the mud surface. Question: Which surface showed less resistance?
Glass