Which of the following is (are) true of ligand-gated ion channels?
a. they are important in the nervous system and they lead to changes in sodium and calcium concentrations in cells.
b. they are important in the nervous system.
c. they are important in the nervous system, they lead to changes in sodium and calcium concentrations in cells, and they open or close in response to a chemical signal.
d. they lead to changes in sodium and calcium concentrations in cells.
e. they open or close in response to a chemical signal.

The following of ligand-gated ion channels are true:

a. They are important in the nervous system and they lead to changes in sodium and calcium concentrations in cells.  

b.They are important in the nervous system.

c. They are important in the nervous system, they lead to changes in sodium and calcium concentrations in cells, and they open or close in response to a chemical signal.

Explanation:

Ligand-gated ion channels (LICs, LGIC) also commonly referred to as ionotropic receptors) are a group of trans-membrane ion-channel proteins which open to allow ions such as Na+, K+, Ca2+, and/or Cl− to pass through the membrane in response to the binding of a chemical messenger, that is, they conduct ion flow through a channel pore in response to the binding of a neurotransmitter. They are different from voltage-gated ion channels, which open in response to voltage (i.e. when the cell gets depolarized) and are sensitive to membrane potentials, and GPCRs, which use second messengers, ligand gated channels open in response to a ligand (some chemical signal) binding to them. Both types of channels are critical for proper activation of the post synaptic neuron.

where’s the list ? the question is incomplete

it's a b and