The phrase "stumbling block" may best refer to ______.

A．The technology for deep underground repositories.
B．A mixture of fuels to meet the electricity needs.
C．The long-term management of nuclear waste.
D．Reducing the resource use to its minimum level.

A．a sub-optimal approach to career development
B．a coherent and credible point of view much needed
C．no money offered to sympathetic politicians
D．academic voice versus industrial interests