C. The Neva River is ... 1) as old as the Littorina Sea. 2) as young as the Tosna. 3) older than Lake Ladoga. 4) younger than the Mga.

Решение:

• В тексте письма говорится: "So, around 2000 BC the Neva River was born! This makes the Neva a very young river."
• Также упоминается, что "Originally, a different river called the Tosna flowed where the Neva is now."
• Следовательно, Невская река моложе Тосны.

Ответ: 2) as young as the Tosna.