What caused virtually all the canada and some of the northern us to be once covered by thick glaciers ice imo’s 15,000 years ago

According to most scientists it was several factors involved. But the main cause was b) changes in the earth's orbit which occurs every 10,000 to 15,000 years. Other contributing factors are changes in the ocean's currents and major shifts in platonic plates of the planet.

Answerweathering;

Your answer is d. silli hope this ! : ) i left an explanation down below if you need it: a sill forms when hot magma squeezes between layers of rock to form a hardened sheet of rock. a sill is basically a tabular sheet intrusion between old layers of sedimentary rock, volcanic beds, or in the direction of foliation inherent in metamorphic rock. the word sill means a concordant intrusive sheet.