[問題] PP test1 Structure Q1

[Kresia]

Most doctors of the Colonial period believed _________ was caused by an imbalance of humors in the body.

a. in disease
b. that disease
c. of disease
d. about disease

我在a跟b猶豫了蠻久的, 有幾各關於名詞子句用法的問題, 想請問大家
1) a 如果原句改成 in (that) disease, 這樣a選項是對的嗎?
2) a 如果像上面這樣改, that後面帶的是that引導的名詞子句對嗎?
3) a 如果這邊把that省略掉 是合法的嗎?

[cocaine]

我覺得不對耶..
in that粗略來看相當於because
所以不能只單獨省略(that)

另外believe that S+V, that似乎不能被省略.
A很明顯錯誤.
B才對.