特伦特承认，在塑料袋中计算出藻类合适的株数，以及在工程系统中使藻类作为一种可替代现有化石燃料的低成本燃料，具有一定的挑战性。
Trent acknowledges that challenges remain in figuring out the right algae strains, and in engineering the system to make algae biofuels a cost-effective alternative to existing fossil fuels.
虽然测试工程师不能估算总的项目成本，但是他们的确可以评估对成本的测试贡献，并且提出测试风险和可行性。
While test engineers cannot estimate total project cost, they can certainly estimate testing contribution to cost, and suggest test risks and feasibility.
该报告警告性地指出了几乎所有地球工程学设想所存在的风险，其可行性、成本、有效性、对环境和社会的影响等方面都存在巨大的不确定性。
The report warned of the great uncertainty about the feasibility, costs, effectiveness and environmental and social consequences of almost all geoengineering ideas.
应用推荐