What do you know about the sea? Some people have seen it, but others haven't. The sea looks beautiful on a fine sunny day and it can be very rough when there is a strong wind. What other things do we know about it?

    Of course, the sea is very large. In the world there is more sea than land. If you swim in the sea, you know that the water is salty. Rivers take salt from the land into the sea. Some places of the sea are more salty than other places. Do you know the Dead Sea? Fish can't live in it!

读图，回答问题。

反比例函数$y=\frac{k-3}{x}$​的图象，当x＞0时，随的增大而增大，则的取值范围是（        ）

已知a,b是实数，则“a>0且b>0”是“a+b>0且ab>0”的                （    ）