听力原文： A Russian cargo ship with about 30 crew members aboard was in danger of sinking off Russia's eastern coast while stormy weather interfered with rescue efforts, state-run Itar-Tass news agency said Friday. Ice had formed on the outside of the ship, and the crew was chipping it off. The crew sent out a distress signal, but there was no explanation of the problem23. Because of the severe weather, aircraft can't be used to rescue the crew. According to Itar-Tass, the vessel is about 90 miles from an oil rig where rescue vessels are based, while a tugboat dispatched from land was still about four times farther away24. The cargo ship had been en route to a fishing area when it experienced problems, the news agency reported. The ship hauls fish from boats to ports.
The Russian cargo ship was sinking. What was the direct cause？

A. No explanation of the problem.
B. Long distance away from land.
C. Slow rescue efforts.
D. Severe weather.