Which of the following might explain the lack of "water worlds"—small planets made mostly of water or other hydrogen compounds—in our own solar system?
a) water worlds can form only on eccentric orbits.
b) there was not enough water in our solar system.
c) in our solar system, the solar wind did not clear out gas until relatively late.
d) small worlds in our solar system formed only close to the sun, where it was too hot for hydrogen compounds to condense.