Hi there: I'm really puzzled by the following question. Hopefully someone can explain it for me. Thanks a lot.

English is not too difficult to master, ____ it gets easier and easier with practice.

a. so b. consequently c. when d. as

The answer is "As". But i don't understand why we couldn't use (a) so or (b) consequently
(a) and (b) have the cause and the effect reversed.
The second clause is supposed to tell why the first clause is true, not the other way around.

I fell down, so/consequently (therefore) I tripped over a tree root. NO

I fell down, as (because) I tripped over a tree root. YES
"as" means that "it gets easier with practice" supports or explains the claim that English is not difficult.

"so" and "consequently" both mean that "it gets easier with practice" is a consequence of the fact that English is not difficult.

In my opinion both meanings are feasible. It could be argued that "as" fits better than "so", but I don't think "so" is wrong. "consequently" has a whiff of a comma splice, which is a reason to dislike that one.
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Ah, yes, you have a point, MrWordy. Either one can be the cause and either one can be the effect. I guess I went with the more likely arrangement, giving the teacher credit for not intending to deliberately bedevil the poor student! [6]

By the way, minwei, welcome to English Forums. Thanks for joining us!

We mustn't leave "when" out of the mix. Things surely tend to be less difficult when they get easier with practice. Emotion: smile Even fronting a non-restrictive clause, "when" can mean "since" in this context.
(Of course things are easy for Jack, when his father owns the company.) Admittedly casual.