Faced with this new situation, people have 30 to find out how to deal with it.
答案：(B) yet
have yet to V 意為「尚未做某事」，本身即帶否定語氣，指人們面對這個新情況還沒找出因應之道；not 無法構成 have not to 這種說法，till 與 though 也接不上不定詞。
yet 你大概只會用在句尾（I haven't finished yet.）。這裡它夾在 have 與 to 中間，句子又看不到 not，所以讀不出否定的意思。
空格夾在 have 與 to find out 之間，要判斷哪一個字接得上不定詞，得看每個選項本身怎麼用。have to 是「必須」，否定要用 don’t have to，英文沒有 have not to 這種寫法，所以 not 填不進去。though 是連接詞，用來連接兩個子句，例如 Though he is young, he is capable.（他雖然年輕，卻很有能力），後面不接不定詞。till ＝ until，是「直到」，後面接時間點或子句，例如 Wait till Monday.（等到星期一）、Wait till he comes.（等到他來），一樣接不上不定詞。三個都排除，只剩 yet。have yet to V 是固定用法，意思是「還沒做某事」，它本身就帶否定，句子裡看不到 not 也不能再加。最快的理解方式是換句話說：have yet to V ＝ still haven’t + p.p.。He has yet to find an investor. ＝ He still hasn’t found an investor.（他到現在還沒找到投資人）。本句 people have yet to find out how to deal with it 就是人們還沒找出因應之道，正好接住前一句的「我們不知道這件事會有什麼結果」。很多人是把 till 看成 still，這個誤讀之所以難察覺，是因為 have still to 確實是英文說法，與 have yet to 同義：He has still to reply. ＝ He has yet to reply.，讀起來完全通順。但選項印的是 till，不是 still——still 是「仍然」，放一般動詞前、be 動詞後，例如 She still lives here.（她還住在這裡）。填進去變成 have till to find out，句子不成立。
not 無法構成 have not to；till 等於 until，後面接時間點或子句；though 是連接詞，後面接子句。三者都接不上不定詞 to find out。
have yet to V（尚未做某事）、be yet to be decided（尚未決定）、as yet（迄今為止）、not yet（還沒）。
It is a usual sunny afternoon in the village of Midwich, England. It seems not 21 any afternoon in the village, but all of a sudden, people and animals lose consciousness. 22 they awake, all of the women of child-bearing age have become pregnant.
This is an episode from a 1960 science fiction story. The women in the story 23 birth to children that have the same appearance. They all have blond hair and "strange eyes." 24 the children grow, they run around the village in a pack, wearing identical clothing and hairstyles, staring at everyone impolitely. 25 one child learns is also known by the others instantly. Villagers begin to 26 their belief that the children all have "one mind."
In this story, the children are produced by some unexplained force from outer space. But this story written 40 years ago 27 predicted the arrival of a recent method of genetic engineering—cloning. Cloning is the genetic process of producing copies of an individual. Will the genetic copies of a human really have "one mind" as 28 in this story? This situation is so strange to us that we do not know what will 29 of it. Faced with this new situation, people have 30 to find out how to deal with it.