<p>First of all, DO NOT use Barron's. Its questions are inaccurate and hence pretty much useless. These types of grammar questions would never appear on the SAT. Stick to the Official SAT Study Guide (latest edition is the second edition), which is written by the same people who make the SAT.</p>
Clause 1: One is radical.
Clause 2: The other is conservative.</p>
<p>When there are two clauses, you can use a coordinating conjunction--like "and"--to tie them together ("one is radical and the other is conservative"). You can also simply divide them into two sentences: "One is radical. The other is conservative." Another alternative is to use a semicolon: One is radical; the other is conservative. A semicolon basically puts 2 individual related statements into ONE sentence. You can't use a comma because a comma can't divide two clauses (or, in this case, two sentences) by itself; you have to either leave them as two separate sentences or use a semicolon.</p>
<p>The reason you can say "one is radical; the other, conservative" and not just "one is radical; the other is conservative" (both are fine) is that you are allowed to omit the second "is." It is implied. This works with a period too: "One is radical. The other, conservative." This concept is not tested on the SAT presumably because it is obscure and not very essential. However, you should know the difference between a semicolon and a period. Here are some examples:</p>
<p>WRONG: The man woke up, he brushed his teeth soon after.
RIGHT: The man woke up and, soon after, he brushed his teeth.
RIGHT: The man woke up and he brushed his teeth soon after.
RIGHT: The man woke up. He brushed his teeth soon after.
RIGHT: The man woke up. Soon after, he brushed his teeth.
RIGHT: The man woke up; soon after, he brushed his teeth.</p>