Many of species of milkweed are among the most dangerous of poisonous plants, while others have little, if any, toxicity.(many 是可以用做代词的呀)
The thirteen stripes of the United States flag represent the original thirteen states of the Union, which they all were once colonies of Britain.(they all)
The bathyscaphe, a free-moving vessel designed for underwater exploration, consists of a Flotation compartment with a observation capsule attached underneath it.(with)
The bathyscaphe, a free-moving vessel designed for underwater exploration, is a Flotation compartment with an observation capsule attached underneath it.