|A) Similar charges of electricity rush towards each other and then get repelled
|B) clouds strike against impurities in air and the friction burns up these impurities
|C) strong opposite charges in different clouds break down the resistance offered by the intervening air
|D) water vapour produces electricity in the clouds
When the resistance of the air between breaks down the current flows from the positive cloud to the negative cloud in a flash which we call lightning . The difference of potential involves is very high.