A Test: Find the Missing Words!

Question

Each of the following pairs of words is missing a word.
Find the missing words.

$$\def\T#1{\text{#1}} \begin{array}{rl} \T{A} & \T{DING}\\ \T{CAN} & \T{GAS}\\ \T{IN} & \T{ADS}\\ \T{LIFE} & \T{LESS}\\ \T{MARSH} & \T{SPICE}\\ \T{OUT} & \T{BACK}\\ \T{PREP} & \T{ANT}\\ \T{PRO} & \T{FISH}\\ \T{SCATTER} & \T{WILL}\\ \T{SEE} & \T{ME}\\ \T{TAX} & \T{LAND}\\ \T{TEE} & \T{REIN}\\ \T{UP} & \T{MAN}\\ \T{WHERE} & \T{EVER}\\ \T{WIN} & \T{HERE}\\ \end{array}$$

 

When you're done with that, ...

There's a word missing from the set of missing words you found.
What's the final missing word?
Why is it missing?

For full marks ...
What kind of test is this?

Answer 1

The final missing word is...

their

Explanation: The missing words for each pair

...create new words when they're linked to each word they're inbetween, thusly:

A Men Ding [Amen, Mending]
Can To Gas [Canto, Togas]
In To Ads [Into, Toads]
Life Time Less [Lifetime, Timeless]
Marsh All Spice [Marshall, Allspice]
Out Come Back [Outcome, Comeback]
Prep Aid Ant [Prepaid, Aidant]
Pro Of Fish [Proof, Offish]
Scatter Good Will [Scattergood, Goodwill]
See The Me [Seethe, Theme]
Tax Is Land [Taxis, Island]
Tee The Rein [Teethe, Therein]
Up Country Man [Upcountry, Countryman]
Where For Ever [Wherefor, Forever]
Win Now Here [Winnow, Nowhere]

And these missing words...

combined are one word ('their') short of a known quote:

"Now is the time for all good men to come to the aid of their country."

Their is probably the odd one out because it's impossible to find a word that ends in 'their' (though there's kinda one that starts with it, 'theirselves'). And it's not in the original sentence, which was a typing exercise by Charles E. Weller ("Now is the time for all good men to come to the aid of the party") and an example of filler text (AKA placeholder\dummy text).