parts per trillion造句怎么写

2、In the case of valeric acid, for example, that concentration was seven parts per trillion-a level only just detectable by bloodhounds.

1、The bees can detect the target chemicals in the air in concentrations as low as a few parts per trillion.

3、Studies have shown that dogs 'noses can detect scents as faint as 500 parts per trillion, making it possible that dogs sense a seizure-indicating pheromone3 prior to an attack.