The handbook is not an easy book to read, but one _____is very useful to anyone who wants to understand the inner workings of this company.

A) that

B) who

C) it

D) how

.

.

(A)

The handbook is not an easy book to read, but _it_is very useful to anyone who wants to understand the inner workings of this company.

2.
Morning traffic in Taipei is always busy with scooters, buses, and cars all _______ for the road.

(A)compete

(B)competed

(C)competing

(D)to compete

.

.

.

.

.

.

.

.

.

.

.

.

.

.

Answer: (C)

和主動被動無關(這裡並不是動詞的分詞轉當形容詞的用法)，而是關係代名的省略，我先還原原句 ：
Morning traffic in Taipei is always busy with scooters, buses, and cars which all compete for the road.
若關代之後無be 動詞，則省略關代之後，再把動詞改為現在分詞。成為：

Morning traffic in Taipei is always busy with scooters, buses, and cars all competing for the road.