【单选题】

The number of permanent crew on the International Space Station will be increased, and possibly doubled, from 2006 by providing it with an extra "lifeboat". The move, announced in Japan last week by the ISS partner nations, means the space station crew will at last be able to do useful scientific research.

The number of crew on the ISS has been limited to three by the capacity of the Russian Soyuz capsules that would return them toEarth in an emergency.But running the ISS requires the full-time attention of more than two crew members, leaving just half the time of one of the crew free for research. This is woefully inadequate, and a NASA-commissioned report concluded in July that no meaningful research is possible with a three-person crew.
NASA、originally planned to replace the Soyuz rescue capsule with a seven-person "crew return vehicle", which would have allowed the ISS to carry a significantly larger permanent crew.But faced with a $ 5 billion budget overrun, the agency cancelled the project last year.
Then last month NASA、announced plans for an orbital spaceplane as a successor to the ageing fleet of space shuttles. This could double as a rescue vehicle for at least six people, but it will not be ready until 2010.
Waiting for the spaceplane could be a big problem as Soyuz capsules have only a six-month lifespan and the Russian agreement to supply them expires in 2006. This could leave a four-year gap with no guaranteed rescue vehicle for the ISS, and hence no crew. Russia has already warned that cash shortages could force it to stop making the capsules.
If the cash can be raised, however, the new plan is to permanently station two Soyuz capsules at the ISS, raising the rescue capacity to six. The ISS’s Russian-built modules have three ports where Soyuz can dock, one of which is usually used for uncrewed cargo modules.
The biggest problem from now to the year 2010 for the ISS lies in that ______.A．Soyuz capsules are not safe enough
B．the ISS will have a 4-year crewless gap
C．more money is needed to maintain the normal operation
D．the ISS partner nations can’t reach agreement during this period