伸{の}びる versus 延{の}びる in a JLPT question

Question

In my JLPT workbook, it gives these examples to explain the difference between 延{の}びる and 伸{の}びる:

髪{かみ}は[１]{いっ}か[月]{げつ}に１センチぐらい伸{の}びる。

会議{かいぎ}はいつも30分{ぷん}ぐらい延{の}びる。

They both mean "stretch", more or less, so I assumed the difference was that 延{の}びる was more about time and space, and 伸{の}びる is more about distance and length.

However, take a look at this practise question:

１９７２年、東京{とうきょう}から大阪{おおさか}までだった東海道{とうかいどう}新幹線{しんかんせん}は岡山{おかやま}まで____________。

A　長引{ながひ}いた 　B　広{ひろ}がった 　C　延{の}びた 　D　伸{の}びた

I chose, D, because it was about length.

According to the book, though, the answer is C.

How can I make sense of this answer in light of the example sentences?

Answer 1

Using the Microsoft IME as a guide, generally 延びる seems to be used as "extend" and 伸びる as "grow":

延びる:

• Extend a conversation
• Extend a schedule
• Extend life span
• Extend a subway into the suburbs

伸びる:

• Plants/people grow
• Expand investigations (in relation to the law etc)
• Grow one's knowledge
• Grow a market/earnings

Answer 2

According to business-textbooks.com

延ばすのもう1つの意味としては、あるものに別のものをくっつけ、長さを足すというものがあります。具体的な物としては、延長コードがこれにあたります。こうした付着による長さの付け足しは、「伸ばす」には含まれませんから、この点は両者の違いとなっています。

So the instance of adding one thing's length to another to extend the total length uses 延, and this seems to match the case in question, that of extending tracks.