You are going to read an article about how animals find their way. Six sentences have been removed. Choose from the sentences A–G the one which fits each gap (1–6). There is one extra sentence you do not need to use.
NATURE'S NAVIGATORS
Every year, countless animals undertake journeys of astonishing length and precision, crossing oceans and continents to reach destinations they may never have seen. 1 How they manage such feats, often with a brain no bigger than a pea, remains one of the great puzzles of biology.
For a long time, the simplest explanation was that animals merely follow familiar landmarks — a coastline, a mountain range, the bend of a river. 2 Many creatures migrate at night, or across featureless ocean, where no such signposts exist.
Researchers now believe that migrating animals carry a whole toolkit of internal instruments. The sun and the stars, for instance, offer a reliable compass to those able to read them. 3 Birds raised in planetariums have been shown to orient themselves by the artificial stars overhead, which suggests the skill is at least partly learned.
Perhaps the most remarkable instrument of all is the ability to sense the Earth's magnetic field. 4 This invisible sense, known as magnetoreception, allows them to establish both direction and, it appears, position. Exactly how they detect the field is still fiercely debated.
Smell, too, plays a role that scientists long underestimated. Salmon, famously, return to the very stream in which they hatched, guided over the final stretch by its unique chemical signature. 5 Remove a salmon's sense of smell, and this homing ability collapses entirely.
What emerges from all this research is a picture of breathtaking sophistication. Rather than relying on any single method, most migrating animals appear to combine several, switching between them as conditions demand. 6 For the scientists who study them, working out exactly how these systems cooperate may take many decades yet.
Removed sentences
A The difficulty is that this cannot possibly be the whole story.
B Some species, it turns out, can perceive it directly and use it like a built-in compass.
C It is a memory of smell, formed in the very first days of life, that draws them home.
D Yet the position of these celestial bodies shifts through the night and across the seasons.
E The result is a navigation system far more reliable than any single cue could provide.
F These remarkable journeys are not aimless wanderings but precisely targeted.
G Most of these animals, remarkably, travel entirely alone, without the guidance of older companions.