19 A)They both attract scientists’ attention
     B)They can both be quite challenging
    C)They are both thought-provoking.
    D)They may both lead to surprising findings.
    20 A)To show how simple the mechanical aids for diving can be.
    B)To provide an excuse for his changeable character.
    C)To explore the philosophical issues of space travel.
    D)To explain why he took up underwater exploration.