The word 'stratum' as used in the passage means

Directions: Read the following passage to answer the given question.

A study recently recognized three varieties of non-native spoken English. Speakers of the first variety exhibit characteristics of a very long and difficult process of internal interpretation from their mother-tongue into English. Thought and expression do not go closely together. A listener would notice this from the way the speaker pauses before expressing himself, apparently deciding which words to use and how to arrange them. Therefore, thinking is exclusively through the medium of the mother-tongue and English is a poor shadow of the original thoughts of the speaker. Utterances heard invariably have to be translated silently into the mother­ tongue to be maximally meaningful, and this makes statements slow, halting and jerky.

The second variety speakers exhibit features described above but to a much lesser degree. Internal translations from the mother-tongue to English still take place during the speaker's expression of opinions and complex concepts but the process now takes a shorter span of time. With ordinary words or simple ideas, internal interpretations need not intervene between the points at which the thought is conceived and the time it is given verbal expression. Discussions are quicker and smoother though these might not always be entirely free from minor halting gaps.
With the third variety, speaker’s thought is almost entirely in English and translation is not needed. The speaker has a wealth of linguistic weapons to choose from and the ability to express the minute distinctions between similar concepts comes quickly and easily. Thus for instance, whereas speakers of the lower varieties have only the word `rain', the speaker of variety recognizes distinctions between `shower', `downpour', `drizzle', 'mist', `deluge', etc., and uses each in the appropriate context. Also, his mastery of the sounds and features of the English Language equips him to express the utterances accurately and automatically without having to pause and think of the correct version of a sound since he masters the various distinctions in the pronunciation of each vowel and each consonant.
We need to recognise, however, that most non-native learners of English `graduate' from one variety to another, from the lowest stratum of variety to the highest stratum of that variety, and then to variety II and so on until they achieve variety III. Length or periods of education, exposure to Standard English and a personal conscious effort play major roles in deciding what variety a particular speaker uses.

Finally, we must recognise that most speakers of the higher varieties are still capable of reverting to the lower ones if they choose to. Thus, we can find even an English language professor switching to the lowest stratum of variety if he feels so inclined when talking to a house-servant or a porter.