I am studying English with Understanding and Using English Grammar by Longman.
In an exercise, I have to use given the time words appropriate to the meaning to combine each pair of sentences.
Directions: Combine each pair of sentences. Use the given time words if they are possible and appropriate to the meaning. Cross out inappropriate ones.
In the new sentences, omit unnecessary words, make any necessary changes (paying special attention to verb forms), and punctuate carefully
Below is the question [Q 17 on page 74]. Either a, b, or c needs to join the sentences.
I will not forget Mr. Tanaka. I will live for a long time.
(a) as (b) as long as (c) so long as
The answer is: b. as long as
I will not forget Mr. Tanaka as long as I live.
I don't understand why the 'c. so long as' cannot be the answer, too? I thought the two words have same meaning.