a.

20 seconds

b.

40 seconds

c.

60 seconds

d.

80 seconds

a.

Their relative masses

b.

Their melting points

c.

The space and forces acting between the molecules

d.

The different molecules in each of them

a.

Switch and an amplifier

b.

Rectifier and an amplifier

c.

Charge store and a switch

d.

Charge store and an amplifier

a.

Momentum

b.

Energy

c.

Electric current

d.

Electric charge