Sentence: Echoes ... for radar to detect distant objects.

[Schwartz]

1.Echoes______for radar to detect distant objects.
a) make it possible b) possibly make c) make possible d) that they make possible

my answer is c) but the correct one is a). I don't understand the reason why it is a).

2.Most dinosaurs were________reptiles.
a) basing on land b) land on which they were based c) based on land d) land-based

my answer is c) but again I chose the wrong one. The correct answer is d)
What does "land-based reptiles" mean?

[Torsten]