<ol>
<li>In the [early days] of the steam locomotive, [compassionate] engineers would sometimes [have thrown] coal overboard [in poor] neighborhoods. [No error]</li>
</ol>
<p>I thought the answer was D because I thought it was more idiomatic to say into poor neighborhoods, but I guess it was wrong. Could anyone please tell me how and why "into poor neighborhoods" is wrong? The answer was C.</p>
<ol>
<li>[Today] a medical doctor must often [make a choice] between [engaging in] private practice [or] engaging in research. [No Error]</li>
</ol>
<p>I chose A because I thought there should be a comma after today. Could someone explain to me why there is no comma after today? Also, In identifying sentence errors, does the SAT test commas as in this sentence? The answer was D.</p>
<ol>
<li>have thrown => throw
in and into are interchangeable in most cases</li>
<li>or => and
not sure how to explain, but there’s nothing wrong with not having a comma after today</li>
</ol>
<p>My sense is that you’re over thinking these two writing questions.</p>
<p>For the first the tense error [(C)] is very serious. The true error is often of a serious nature. If you miss the true error you can easily get distracted by perceived flaws in the other choices. Test takers find the “no error” choice difficult to make.</p>
<p>There is in my mind a distinction between “in neighborhoods” and “into neighborhoods”, and that in principle both could be right. In the case of “into”, consider something like “throw coal overboard into the ditch”. The use of “in” suggests the general location or a general practice, as “throw coal overboard in California”.</p>
<p>For the second, the usage is "between … and … ", and “either … or …”. There isn’t a permissible alternative.</p>
<p>Commas are often optional, especially with short clauses like the one word “Today”. But in a sentence with an even longer introductory clause, such as "In a world that’s becoming increasingly complex [,] a medical doctor … " the comma is unnecessary. If you as the writer wants to force a break then you can use it. All this said, the SAT rarely tests for commas, except for serious misuse such as when they are used to tie two sentences together.</p>