John plays football_________,if not better than ,David.

B. as well as

John plays foot ball as well as, if not better than, David.
如果John的football 玩得没David好，那就是跟他一样好。

如果没有后面那一句
John plays football （A) as well. 意思就是 John 也玩 football.
John plays football (C) so well. 意思是 John 的 football 玩得很好。
John plays football (D) so well as..... so well as 不是一个明确的短语。

只有(B) as well as 是正确。
因为在这里，John 被用来和David作比较。
"as well as" 意思就是 跟____一样好

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B 答案
意思是 约翰踢球即使不比大卫强， 也是差不多，
就是最起码水平相当， 或者比大卫踢足球更好的意思
A as well 只有 “也“的意思 ， 例如， John is a student, David is as well.
B as well as + 名词 或代词 John plays football as well as David, 中间那句话是插入语
C so well 不对
D so well as 用于否定句 John doesn't play football so well as David. 约翰不如大卫踢球好

B. as well as, 一样好， 如果不是更好的话