Mountain ranges were a big problem because in the winter there were avalanches, it was very cold, and there was a lot of snow.
Answer: D.) They study the stars and gases around black holes.
Explanation: Given the technology that we have in the 21st century (as of 2020), scientists cannot study black holes directly, and must resort to observing and detecting how the presence of them exists by noting how they take an effect on the matter surrounding them.
The correct answer is present progressive. The present progressive is used for an event that continues to occur at the present moment in time. Thank you for posting your question. I hope this answer helped you. Let me know if you need more help.
