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I understand that pace and speed are often interchangeable, however, why are they not in this case? Only the latter is correct according to the answer key.

The lifeguards were on a training exercise in the sea when the dolphins swam towards them at considerable speed, then circled them repeatedly, hitting the surface of the water with their fins.

Here are the Cambridge definitions: pace speed

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  • All I can say is "the answer key" is just someone's opinion - with which I disagree. Perhaps the test setter is a bit literal-minded, and thinks that since dolphins don't have legs, they can't move at a pace. In which case I'd just offer The kid got into his ambulance and drove away at a fair pace. – FumbleFingers May 2 '18 at 13:43
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    "a considerable pace" would seem more natural to me (BrE). That might be the deciding factor? – Phil M Jones May 2 '18 at 13:45
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The reason speed and pace aren't always interchangeable is that speed is fundamentally a more abstract noun.

Fast is an adjective for an object's quickness of motion. A pace can be a step or an object's rate of movement (in other words, a speed). But while speed can also be a rate of movement, in its more general sense it refers to an abstract concept that is a scalar (continuously variable by an attached magnitude) quantity of motion. For example, one pace can be faster than another, and one speed can be faster than another, but an object cannot have more or less pace as it could more or less speed.

So when the sentence says, "…at considerable speed," it refers to an object moving at significant [amounts of our abstract concept that is speed]. But that does not equivalently mean the object is moving at significant [amounts of pace], because "pace" already refers to a particular amount of speed, and not to the general concept the quantity is. However, if the sentence had a determiner such as a in "…at [a] considerable speed," speed and pace would be interchangeable, because "considerable" would then be describing an attribute of a particular speed or pace, and not the extent or degree of the concept present.

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