One of the main advantages of hydrogen as a fuel is that:

a) plants can economically produce the hydrogen needed
b) it exists as a free gas
c) it can be economically supplied by the world's ocean
d) the only product of hydrogen combustion is water
e) it contains a large amount of energy per unit volume of hydrogen gas

The correct answer is option D.

Explanation:

A fuel cell is defined as the electrochemical cell which converts the chemical energy of a fuel (often used hydrogen) and an oxidizing agent (often used oxygen) into electrical energy via a pair of redox reactions.

The reactions which occur in hydrogen-oxygen fuel cell are:

At cathode:  

At anode:  

As we can see that reaction of hydrogen gas with oxygen gas gives us the water as a product which can be used for other things and it also doesn't pollutes environment.

i am going to put the answers in order i dint feel like typing it out so yeah

a

d

c

b

b

b

a

d

a

b

c

c

c

b

d

so here we go

atomic mass

239 + 1 = y + 134 + 3 combine like terms

240 = y + 137             subtract 137 from both terms

240 - 137 = y

103 = y

atomic number

94 = z +54 + 0

94 = z + 54               subtract 54 from both sides.

40 = z

answer