A．They were caused by the dirty smoke from burning coal.B．They were so thick that you could only see your own hand in front of you.C．They are still characteristic of London as London Particulars .D．They are the symbol of the mystery of London.
A．They were caused by the dirty smoke from burning coal.
B．They were so thick that you could only see your own hand in front of you.
C．They are still characteristic of London as "London Particulars".
D．They are the symbol of the mystery of London.