在强酸性溶液中能大量共存的离子组是（　　）

A．Ca²⁺、Na⁺、NO₃^-、CO₃^2-

B．Mg²⁺、Cl^-、NH₄⁺、SO₄^2-

C．K⁺、Cl^-、HCO₃^-、NO₃^-

D．Ca²⁺、Na⁺、Fe²⁺、NO₃^-

The scientists have made _______ possible for us to communicate with each other more easily.

A．it

B．that

C．this

D．them