The weather in Beijing is colder than that in Nanjing.
2. choose the b object clause. The leading words don't have this and these, so they are directly excluded. Judging from the meaning of the question, the clause part itself is a declarative sentence, which leads to the second usage of that.
On the object clause: several problems that should be paid attention to in the object clause;
1. Word order: it must be the word order of a declarative sentence. However, it should be noted that what's wrong/the matter/after to you and other sentences seem to question the word order, but in fact they are statements.
2. About words with sounds:
If the clause itself is a declarative sentence, use a declarative sentence.
If the clause itself is a general question, use if or where. When there is .. or not at the end of the sentence, you can only use where.
If the clause itself is a special question, use your own special question.
3. About tenses
The main clause is the present tense, and the clauses are unrestricted, so you should use everything. The main clause is the general past tense, and the clauses use one of the corresponding past tenses. Note: When clauses express objective truth and eternal laws, they use the simple present tense regardless of the tense of the main clause.