Are things around us usually made of pure elements or combinations of elements?

Things around us are usually made up of combinations of elements. These combinations are called molecules. Molecules make up combinations called compounds.

Water is an example. There are 2 oxygen atoms and 1 hydrogen atom (oxygen and hydrogen are elements). Together there molecules make up H2O (water) which is a compound.

I hope this makes sense! I'm happy to answer any other questions you might have :)

option a is correct i.e.

only sodium ions can pass through the membrane.

explanation:

in context to the membrane of nerve impulse, the outer membrane of neuron is semi permeable that is only sodium ions can pass through the membrane i.e. smaller ions can pass through the membrane while larger ions cannot pass through it.