To protect itself from herbivores, plants
a)cannot do anything because they cannot protect themselves
b)both secure toxins and have thorns
c)secrete toxins
d)have thorns

B) both secrete toxins and have thorns

Plants undergoes several adaptations in order to survive in their environment and to defend themselves from the herbivores. The presence of thorns on the surface of the plants is a structural adaptation, these thorns will get inserted in the herbivores and will discourage their feeding on the plants. They secrete toxins to the external surface, it is physiological adaptation, which can kill the herbivores and also discourage other members from feeding.

estuaries are the partially enclosed coastal bodies of brackish water formed at the mouth of sea where river meets an ocean. it is a transition zone between river and sea water so estuaries are influenced to both marine environment such as tides, waves and to riverine such as flow of fresh water and sediments. the mixing of sea and fresh water provide high level of nutrients, making estuaries a high product habitats.

many estuaries suffers degradation from a variety of human activities like erosion from deforestation, overgrazing, overfishing, drainage and eutrophication.

so from the above statement answers will be

1. false as estuaries are formed due to both brackish and freshwater.

2.true, due to high productivity estuaries are breeding ground for many marine species.

3. direct and indirect activities of man that contribute to pollution and deposition of sediments.