Ik it’s either b or d

Geologists know that potassium-40 decays to argon-40, with a half-life of 1.3 billion years. if an analysis of a feldspar crystal in a sample of granite shows that 75 percent of the potassium-40 atoms have decayed to form argon-40, what is the age of the granite

What are the answers for the following questions?

Which of the following is the major contributing factor in all landslides? stream erosion precipitation earthquakes gravity sedimentary layers oriented parallel to the slope